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electrical 

Instruments and Testing 



HOW TO USE THE VOLTMETER, OHMMETER, AMMETER, 

POTENTIOMETER, GALVANOMETER, THE WHEATSTONE 

BRIDGE, AND STANDARD PORTABLE TESTING SETS 



By 
NORMAN H. SCHNEIDER 

Author of il The Care and Management of Electric Power Plants^ "Induction Coils 
and Coil Making" M Electrical Circuits and Diagrams" etc., etc. 

WITH NEW CHAPTERS 

ON TESTING WIRES AND CABLES 

AND 

LOCATING FAULTS 

By 
JESSE HARGRAVE 

Assistant Electrical Engineer Postal Telegraph Cable Company 



Third Edition, Revised and Considerably 
Enlarged, with 28 New Diagrams 



NEW YORK: 
SPON & CHAMBERLAIN, 123-125 Liberty Street 

LONDON : 
E. & F. N. SPON, LIMITED, 57 Haymarket, S. W. 

1907 



LIBRARY of CONGRESS 
Two Copies Received 

DEC 16 1907 

Copynf nt fcntry 
July A £<J ft of 
CLASS/? XXc, No. 

COPY S. 



?v 



Copyright, 1904 
By SPON & CHAMBERLAIN 

Copyright, 1907 
By SPON & CHAMBERLAIN 



Entered at Stationers' Hall, London 



BURR PRINTING HOUSE 
NEW YORK 






i .-/:■ ■ I 



PREFACE TO THIRD EDITION- 



Since this work was first published it has 
found a ready sale among telegraph and telephone 
workers, and it was pointed out that the testing of 
telegraph and telephone wires was not as fully 
covered as it might have been. With a view to 
making the book still more attractive and useful 
to workers in those important branches of the elec- 
trical field, Mr. Jesse Hargrave, Assistant Electrical 
Engineer of the Postal Telegraph Cable Company, 
was asked to contribute two chapters devoted to the 
testing of telegraph and telephone wires and cables, 
and he agreed to undertake the work. 

In compiling the matter contained in Chapters 
XII and XIII Mr. Hargrave has endeavored to 
carry out Mr. Schneider's ideas of clearness and 
simplicity. The testing of the wires is taken up 
in its first stage — the first test of the day, the early- 
morning test — and the student is taken step by 
step through the different methods of testing for 
faults, insulation and conductivity, cable faults, etc. 



IV PREFACE 

The different tests and measurements, aside from 

being described in the simplest possible language, 

are treated from a strictly practical standpoint, Mr. 

Hargrave having made use of them in actual 
practice. 

Thanks are due Messrs. Leeds & Northrup for 
sketches, etc. 



PREFACE, 



This book is intended for practical use and 
also as an introduction to the larger works on 
Electrical Testing. 

The apparatus described is modern and uni- 
versally adopted. 

The tests are such as occur daily in the work 
of the engine room, power house or the tech- 
nical school. 

The illustrations except those of instruments by 
specified makers are drawn for this book. Detail 
unnecessary to the subject is omitted, such can be 
well studied in the apparatus itself. 

The formulas given are explained in the text 
preceding them, and examples are worked out in 
simple arithmetic. Formulas are often looked 
upon as intricate mysteries by those who do not 
understand them. In reality they are often very 
simple and very necessary. The use of formulas 
is but a method of arithmetical shorthand. A few 
minutes devoted to a formula will render it clear 
and its use easy to those of even limited mathe- 
matical education. 



VI PREFACE. 

The thanks of the author are due to Mr. 0. T. 
Louis for cuts and suggestions, to Mr. F. W. 
Roller for information on his improved type of 
hot wire instruments; to Messrs. Willyoung & Gib- 
son, The Weston Electrical Instrument Co., The 
Keystone Electrical Instrument Co., The General 
Electric Co., and others. 



CONTENTS. 



CHAPTER I. 

The simple galvanometer. Deflections not propor- 
tional to current. Ampere turns. Selection of 
size of wire for coil. Tangents. The tangent 
galvanometer. Influence of the earth on a 
galvanometer. The astatic galvanometer. 
Compensating magnet 1 

CHAPTER II. 

Sensibility of galvanometer. Figure of merit or con- 
stant. The Thompson reflecting galvanometer. 
Forms of the D' Arson val reflecting galvanometer. 
Ballistic galvanometers 12 

CHAPTER III. 

Rheostats. Resistance wires and their composition. 
Laboratory resistance slab. Shunts. Condens- 
ers. Keys. The reversing key. The Rymer- 
Jones key. Commutator. The Kempe discharge 
key. The standard cell. Clark cell. Weston 
cell 23 

CHAPTER IV. 

The voltmeter. The series ammeter. The shunt 
ammeter. Types of instruments. Sensibility. 



viil CONTENTS. 

Duplex instruments. Potential indicators. Milli- 
volt and milliampere. Multipliers. Hot wire* 
instruments. Shunts. The wattmeter. Thom- 
son inclined coil instrument. Queen instru- 
ments. Keystone instruments. G. E. potential 
indicator. Electrostatic voltmeters. Electro 
dynamometer type. Electromagnetic type. 
Reading instruments. Parallax. Care of 
instruments 39 

CHAPTER V. 

The Wheatstone bridge. Post Office bridge. How to 

use the bridge 75 

CHAPTER VI. 

Forms of portable sets and how to use them. Queen 
bridge. Willyoung bridge. Whitney bridge. 
Sage ohmmeter. Evershed testing set 85 

CHAPTER VII. 

Current flow and e.m.f. Galvanometer constant. 
Direct deflection method. With Queen set. 
With voltmeter. Testing resistance of galvano- 
meter. Five methods of battery testing 102 

CHAPTER VIII. 

The Potentiometer. Checking voltmeter Checking 

e.m.f. Use of various portable testing sets. . . . 123 

CHAPTER IX. 

Charge and discharge of condenser. Testing capa- 
city and insulation of condenser. Loss of charge 
method 142 



CONTENTS. IX 

CHAPTER X. 

Cable testing. Capacity. Insulation and conductivity. 
Locating cable faults. Varley test. Murray 
test 135 

CHAPTER XI. 

Testing with voltmeter. Testing wiring. Insulation 
of generator. E.m.f. around commutator. 
Measuring drop. Testing high e.m.f. with low 
reading voltmeter. Temperature and resistance. 
Testing temperature by rise of resistance. Test- 
ing field coils. Testing armature coils. Plot- 
ting curves of tests 163 

CHAPTER XII. 

Early morning tests. Wrecks: Locating grounds by 
Wheatstone bridge measurements. Measure- 
ment for crosses using Varley test. Measure- 
ment for crosses using the two cross wires only. 
Locating a cross by voltmeter test. Insulation 
tests — milli-ammeter method. Insulation tests 
by voltmeter method. Conductivity tests 193 

CHAPTER XIII. 
Location of grounds and crosses by Varley method 
using multiplied arm ratios. How to find trouble 
after located. Varley test by Leeds & Northrup's 
dial testing set. Murray test with Leeds & 
Northrup dial bridge. Locating openings in cable 
conductors by bridge method. The Leeds & 
Northrup fault finder. Resistance measurement 
with the Leeds & Northrup fault finder. Fault 
location with the Leeds & Northrup fault finder. 
To locate openings using buzzer and telephone.. . 211 



LIST OF ILLUSTRATIONS. 



1 Simple galvanometer 1 

2 Tangent 15° 7 

3 Tangent 30° 7 

4 Tangent 60° 7 

5 Tangent 75° 7 

6 Astatic galvanometer 9 

7 Compensating magnet 10 

8 Tripod galvanometer 13 

9 Four coil galvanometer 15 

10 D'Arsonval galvanometer 10 

11 D'Arsonval galvanometer 18 

12 Wall pattern galvanometer 19 

13 Willyoung suspension 20 

14 Willyoung portable galvanometer 21 

15 Galvanometer scale 22 

16 Rheostat plug switch 24 

17 Shunt and galvanometer 28 

18 Condenser 30 

19 Condenser built up 30 

20 Condenser adjustable 31 

21 Reversing key 33 

22 Rymer Jones key 34 

23 Commutator 35 

24 Kempe discharge key 36 

25 Weston standard cell 38 

26 Voltmeter connection 40 

27 Series ammeter 41 



X 



LIST OP ILLUSTRATIONS. xi 

28 Shunt ammeter 41 

29 Weston voltmeter detail 42 

30 Weston portable instrument 43 

31 Weston laboratory instrument 44 

32 Laboratory instrument scale 44 

33 Switchboard instrument 45 

34 Switchboard instrument flush type 46 

35 Switchboard instrument illuminated dial 47 

36 Switchboard instrument illuminated dial 47 

37 Duplex instrument 48 

38 Potential indicator scale 50 

39 Multipliers 52 

40 Styles of instruments 53 

41 Whitney hot wire system 55 

42 Whitney portable instrument 56 

43 Whitney switchboard instrument 57 

44 Weston shunts 58 

45 Whitney shunts 59 

46 Solenoid type 61 

47 Solenoid type 61 

48 Magnetic vane type 61 

49 Magnetic needle instrument 62 

50 Electromagnetic instrument 62 

51 Thermometric instrument 62 

52 G. E. potential indicator 63 

53 Thomson inclined coil ammeter 64 

54 Thomson switchboard ammeter 65 

55 Thomson edgewise voltmeter 66 

56 Thomson astatic voltmeter 67 

57 Edgewise voltmeter 72 

58 Kite or lozenge bridge 76 

o9 P. O. bridge 78 

60 Willyoung bridge 86 

61 Willyoung bridge diagram 87 

62 Queen Acme set 89 

63 Whitney testing set 92 

64 Whitney testing set diagram 92 



xil LIST OF ILLUSTRATIONS. 

65 Slide wire bridge 95 

66 Sage ohmmeter • 97 

67 Sage ohmmeter diagram 98 

68 Sage ohmmeter diagram 98 

69 Evershed testing set 99 

70 Evershed testing set 100 

71 Fall of potential diagram 103 

72 Direct deflection method. 105 

73 Setting of Queen bridge Ill 

74 Direct deflection with voltmeter 114 

75 Resistance of galvanometer 115 

76 Resistance of galvanometer 116 

77 Resistance of battery 117 

78 Resistance of batter)' by bridge 120 

79 Potentiometer 124 

80 Willyoung potentiometer 131 

81 Charging condenser 135 

82 Discharge of condenser 136 

83 Telephone test of cable 145 

84 Testing cable 1 46 

85 Circuit for cable test 14S 

86 Cable test with Aone set 150 

87 Cable test with Aone set 151 

88 Testing for fault 155 

89 Testing for fault 156 

90 Varley test 1 .",7 

91 Murray test 158 

92 Murray test with Aone set 161 

93 Testing resistance with voltmeter 164 

94 Insulation of generator 165 

95 E.m.f. around armature 173 

96 E.m.f. around armature 173 

97 Drop acros's lamp 174 

98 Hign e.m.f. with low reading voltmeter 176 

99 Testing field coil ISO 

100 Testing field coil 180 

101 Testing field coil 181 



LIST OF ILLUSTRATIONS. xill 

102 Testing armature 183 

103 Testing armature 184 

104 Testing drop on line 185 

105 Plotting curves 188 

100 Relay test for a "ground" 104 

107 Relay test for an opening 104 

108 Relay test for a cross 105 

100 Relay test-out of a wreck 106 

110 Automatic signaling device to indicate when wires 

come O.K 106 

111, 112 and 113 Measurements for crosses using Varley 

test 100, 200 

114 Measurement for crosses using the two crossed 

wires only. First method 202 

115 Measurement for crosses using the two crossed 

wires only. Second measurement 203 

116 Another method of locating a cross 204 

117 and 118 Locating a cross by voltmeter test. .. .205, 206 

110 Insulation tests. Milli-ammeter method 207 

120 and 121 Location of cable fault by Varley method, 

using multiplied arm ratios 212, 213 

122 Check measurement 214 

123 Method of finding trouble after locating it 216 

124 Theoretical arrangement of Leeds & Northrup 

dial bridge 218 

125 Actual arrangement of Leeds & Northrup dial 

bridge 210 

126 Locating opening in cable conductor by bridge 

measurement 220 

127 Locating opening in cable conductor by bridge 

measurement. Second method 221 

128 Resistance measurement with the Leeds & North- 

rup fault finder 224 

120 Fault location with the Leeds & Northrup fault 

finder. First method 225 

130 Fault location with the Leeds & Northrup fault 

finder. Second method 226 



XIV LIST OF ILLUSTRATIONS. 

131 Fault location with the Leeds & Northrup fault. 

finder. Third method 227 

132 To locate opens using buzzer and telephone by em- 

ploying Leeds & Northrup fault finder 227 

133 Actual arrangement of Leeds & Northrup fault 

finder 2»« 



CONTENTS OF TABLES. 



I. Resistance of Copper Wire 5 

II. Natural Tangents of Angles 8 

III. Setting of Bridge Arms 91 

IV. Specific Inductive Capacity 141 

V. Voltmeter readings and Resistance Tests 171 

VI. Increase of Resistance with Temperature 177 



INTRODUCTION, 



Classification. Electrical instruments used in 
the detection or measurement of electrical currents 
are divisible into two classes. 

The first class includes such as show the pres- 
ence of a current without directly indicating its 
value in units of measurement. 

To this class belong the various kinds of gal- 
vanometers. 

Instruments of the second class indicate by the 
movement of pointers over dials or some such 
means the value of the electrical current passing. 

The dial under the pointer or index is graduated 
into divisions representing units of measurement. 
Examples of this class are the voltmeter, ammeter 
and wattmeter. 

Instruments which leave a record of the fluc- 
tuations or the current strength for a given period 
of time are Recording Voltmeters, Recording Watt- 



XVI INTRODUCTION. 

meters, etc., according to the unit by which their 
records are read. 

As the underlying principles of instruments be- 
longing to the latter class are mostly to be found 
in those of the first class, the latter will be first 
considered. 



CHAPTER I. 

Galvanometers. 

The Simple Galvanometer. Perhaps the simplest 
form of galvanometer is shown in Fig. 1, and al- 
though home made, is capable of many uses. 




Fig. 1. 

An ordinary mariner's compass has a few turns 
of fine insulated copper wire wound around its 
case, a suitable gauge being No. 26 B. & S. or No. 
28 S.W.G. 

If current from a battery or other source be 
sent through the coil of wire, the magnetic needle 
will tend to turn at an angle to the wire turns, 
providing the galvanometer has first been set so 
that both needle and wire turns are parallel. 

l 



2 ELECTRICAL INSTRUMENTS. 

The stronger the current through the coil, the 
farther the needle swings out from it. 

And reversing the connections will cause the 
needle to swing in a reversed direction as the cur- 
rent is reversed in its flow around the coil. 

But it will be noticed that the amount of the 
needle swing is not in proportion to the strength 
of the current. 

Let the circle or card forming the bottom of 
the compass case be divided into 360 degrees 
having the zero where one end of the needle nor- 
mally rests. 

On connecting the coil ends to a dry cell suppose 
the needle swings out to 30 degrees. 

If a second cell is put in series with the first 
and the two cells connected to the coil as before 
the swing may now only be 35 degrees, a gain of 
but five degrees by doubling the current strength 
instead of a gain of 30 degrees. 

It is to be remarked before proceeding that 
these readings as they are termed are only for 
sake of example, not being necessarily exactly 
the number of degrees obtained in any experiment. 

If a galvanometer gives a large reading with one 
cell as compared with a reading taken under pre- 
cisely similar conditions on another galvanometer 
the one giving the largest reading will be said to 
have the greater sensibility. 

The movement of a galvanometer needle from 
one position to another is termed its deflection 
and is measured in degrees. 



GALVANOMETERS. 3 

Now the primary laws of electricity teach that 
the current strength or rate of flow depend upon 
the electromotive force (e.m.f.) or voltage im- 
pelling that current and upon the resistance of 
the circuit through which it travels. 

Increasing the e.m.f. or decreasing the resist- 
ance increases the current flow, and vice versa, 
decreasing the e.m.f. or increasing the resistance 
decreases the current flow.* 

In order to decrease the deflection of the needle 
resistance must be added to the circuit, this may 
be done by means of a rheostat. 

One form of rheostat v^ould consist of coils of 
wire so connected that one or any desired number 
of coils could be inserted in series with the gal- 
vanometer. The electrical energy expended in 
these coils being therefore not available in the 
galvanometer coils would have no effect upon its 
needle deflection which would be thereby reduced. 

The sensibility of this galvanometer will depend 
upon the ampere turns of its coil (neglecting fric- 
tion, etc.), and the coil winding should be suitable 
for the current in the circuit in which it is used. 

If the galvanometer is to measure currents in 
a circuit of low resistance, its coil should be also 
of low resistance and vice versa. 

The meaning of ampere turns must be thor- 
oughly understood. It will suffice here to remark 
that one ampere traveling round the needle for 

*See " Electricity and Its Laws, for Beginners," for the 
elementary laws governing electric current. 



4 ELECTRICAL INSTRUMENTS. 

one turn would equal one hundredth of an ampere 
for one hundred turns and the needle deflection 
would be the same in each case. 

In a circuit of low resistance, a large current 
would flow (presumably) and this current would 
not need to travel around the needle as many 
times as the lesser current in a circuit of higher 
resistance to produce the same deflection. In this 
case using a coil of many fine wire turns in the 
low resistance circuit would only reduce the cur- 
rent around the needle. And using a coil of large 
wire and few turns would not give ampere turns 
enough in a circuit of high resistance. 

An apparent contradiction may be met by the 
experimenter who in adding more turns of wire 
to a simple galvanometer gets a greater deflection 
of the needle. This, however, only shows that 
the ampere turns were not sufficient in the first 
case, and therefore the contradiction does not exist. 

To sum up, the reason for using a large number 
of fine wire turns as is done in some galvanometers 
is that the current strength is often so small as 
to require its circulation many times around the 
needle to produce the requisite ampere turns. 

It is very common practice to speak of a coil 
as being wound to a given resistance when ampere 
turns are really desired. If resistance were the 
object, a resistance metal could be used to advan- 
tage. 

The relation of resistance and diameter of cop- 
per wire is given in Table I., from which may be 



GALVANOMETERS. 



TABLE I. 
Table Showing Gauge Diameter, Area and Resist- 
ance of Copper Wire. 





Brown and Sharpe 


Gauge. 


Standard Wire Gauge. 


Gauge 
No. 


Dia- 
meter 
inches. 


Circular 
Mils. 


Ohms per 
1000 ft. at 
15° Cent. 


Dia- 
meter 
inches. 


Circular 

Mils. 


Ohms per 
1000 ft. at 
15° Cent. 


0000 


.4600 


211600 


.048 


.400 


160000 


.063 


000 


.4096 


167810 


.060 


.372 


138384 


.073 


00 


.3648 


133080 


.076 


.348 


121104 


.084 





.3249 


105590 


.096 


.324 


104976 


.096 


1 


.2893 


83694 


.121 


.300 


90000 


.113 


2 


.2576 


66373 


.153 


.276 


76176 


.133 


3 


.2294 


52633 


.193 


.252 


63504 


.160 


4 


.2043 


41743 


.243 


.232 


53824 


.189 


5 


.1819 


33102 


.307 


.212 


44944 


.226 


6 


.1620 


20250 


.387 


.192 


36864 


.276 


7 


.1442 


20817 


.489 


.176 


30976 


.329 


8 


.1284 


16510 


.616 


.160 


25600 


.397 


9 


.1144 


13094 


.777 


.144 


20736 


.490 


10 


.1018 


10382 


.980 


.128 


16384 


.624 


11 


.0907 


8233.7 


1.236 


.116 


13456 


.758 


12 


.0808 


6530.3 


1.559 


.104 


10816 


.940 


13 


.0719 


5178.2 


1.966 


.092 


8464 


1.202 


14 


.0640 


4106.2 


2.479 


.080 


6400 


1.590 


15 


.0570 


3255.8 


3.127 


.072 


5184 


1.963 


16 


.0508 


2582.7 


3.942 


.064 


4096 


2.485 


17 


.0452 


2047.6 


4.972 


.056 


3136 


3.246 


18 


.0403 


1024.9 


6.269 


.048 


2304 


4.420 


20 


.0319 


1022 


10.140 


.036 


1296 


8.038 


22 


.0253 


642 


16.120 


.028 


784 


13.212 


24 


.0201 


404 


25.630 


.022 


484 


21.394 


26 


.0159 


251 


40 . 750 


.018 


324 


31.963 


28 


.0126 


160 


67.790 


.015 


219 


47.707 


30 


.0100 


100 


103.300 


.012 


154 


66.866 


32 


.0080 


63 


183. 


.011 


106 


103.410 


34 


.0063 


40 


291. 


.009 


84 


127.860 


36 


1.0050 


25. 


|462. 


.008 


1 57 


189.660 



6 ELECTRICAL INSTRUMENTS. 

computed the size of wire to use for a galvanometer 
coil. 

Deflections not Uniform. The needle does not 
move twice as far for twice the current because 
as soon as it starts to move it commences to get 
out of the strongest influence of the coil, or out of 
the intense part of the field. 

If the coil is very large and the needle very 
small, then the latter can deflect more uniformly 
as the field is more uniform. 

Tangent Galvanometer. The tangent galvano- 
meter has a coil wound around a ring perhaps 
twelve inches in diameter, and the needle is a 
fraction of an inch in length. 

In this case the deflections of the needle are 
compared by referring to a table of tangents 
Table II. 

A tangent is a straight line perpendicular to the 
diameter of a circle, limited in length by the sides 
of the angle one of which is the diameter of the 
circle. 

In Fig. 2 the dotted line is the diameter of the 
circle and passes clear through the zero mark. 
N is a straight line drawn through the centre of 
the pointer or needle, in other words shows the 
number of degrees of deflection. The degrees be- 
tween this line and the zero will be the number 
of degrees forming the angle of deflection, angles 
being measured in degrees. If the pointer were at 



GALVANOMETERS. 7 

45° the angle would be one of 45° or half a right 
angle (90°). 

T is a straight line perpendicular to the diame 
ter but cut by N. The portion between the diame- 
ter line and where it is cut by N is the tangent. 
Fig. 2 represents an angle of about 15°, that is, the 
needle is deflected 15°. Fig. 3 shows the tangent 
of an angle of about 30°, Fig. 4 the tangent of 60°. 
and Fig. 5 the tangent of about 75°. 

It will be seen that the tangent increases in 




Fig. 2. 



Fig. 3. 



Fig. 4. 



Fig. 5 



length very rapidly, far more so than the deflec- 
tion angle. 

To make a comparison, let the needle be de- 
flected 5°, its tangent will be .0875, double the 
deflection, the tangent of 10° is .1763, more than 
double the one of 5°. 

Deflect the needle to 30° or six times its first 
deflection and the tangent is .5774, about seven 
times. 

A still farther deflection to 60°, or twelve times 



8 



ELECTRICAL INSTRUMENTS. 



the first one, the tangent is now 1.7321, or about 
twenty times that of 5°. 

In a tangent galvanometer when it is desired 
to compare the strength of two currents, readings 



TABLE II. 
Natural Tangents of Angles. 



Angle 


Tangent 


Angle 


Tangent 


Angle 


Tangent 


Degrees 




Degrees 




Degrees 




1 


.0174 


31 


.6008 


61 


1.8040 


2 


.0349 


32 


.6248 


62 


1.8807 


3 


.0524 


33 


.6494 


63 


1.9626 


4 


.0099 


34 


.6745 


64 


2.0503 


5 


.0874 


35 


.7002 


65 


2.1445 


6 


.1051 


36 


.7265 


66 


2.2460 


7 


.1227 


37 


.7535 


67 


2.3559 


8 


.1405 


38 


.7812 


68 


2.4751 


9 


.1583 


39 


.8097 


69 


2.6051 


10 


.1768 


40 


.8391 


70 


2.7475 


11 


.1943 


41 


.8692 


71 


2.9042 


12 


.2125 


42 


.9004 


7'2 


3.0777 


13 


.2308 


43 


.9325 


73 


3.2709 


14 


.2493 


44 


.9656 


74 


3 . 4879 


15 


.2679 


45 


1 . 0000 


75 


3.7321 


16 


.2867 


46 


1.0355 


70 


4.0108 


17 


.3057 


47 


1.0712 1 


77 


4.3315 


18 


.3249 


48 


1.1106 


78 


4 7046 


19 


.3443 


49 


1.1504 


79 


5.1446 


20 


.3639 


50 


1.1918 


80 


5.6713 


21 


3838 


61 


•.2349 


81 


6 3138 


22 


.4040 


62 


i 2799 


°2 


7 1154 


23 


4244 


53 


I 3270 


83 


8.1443 


24 


4452 


54 


1.3704 


84 


9.5144 


25 


.4663 


55 


1.4281 


85 


11.4301 


26 


.4887 


56 


1 . 4826 


86 


14.3007 


27 


.5095 


57 


1.5399 


87 


19.0811 


28 


.5317 


58 


1.6003 


88 


28.6863 


29 


.5543 


59 


1 . 6643 


89 


57.2900 


30 


.5773 


60 


1.7321 


90 


infinite 



are made of the two deflections obtained. The 
tangents of these angles or deflections are then 
compared. 

If one deflection were 11° and the second one 



GALVANOMETERS. 9 

63°, the second current would be nearly ten times 
the greater, or as .1944 is to 1.9626. 

Influence of the Earth's Magnetic Field. It was 

necessary to move the galvanometer around so as 
to bring the needle point to zero. 

As the galvanometer was made out of a com- 
pass this was done because the needle tended to 
point to the magnetic N and 5 of the earth. 

In order to avoid having to turn the instrument 
about before using it, the influence of the earth's 



C_N: 



€ 



3 



iKZ) 



Fig. 6. 

magnetic field is compensated for in the astatic 
galvanometer. 

The Astatic Galvanometer. In Fig. 6 two mag- 
netic needles are rigidly attached to a stem in 
such manner that the N pole of one is over the 5 
pole of the other. If both needles are equally 
strong, such a combination on being suspended 
by a fine thread T or balanced on a point, would 
stay in whatever direction it was placed. It 
would be entirely unaffected by the earth's mag- 



10 



ELECTRICAL INSTRUMENTS. 



netic field, one needle neutralizing the other. But 
this is not obtained in practice, the pair of needles 
take a set position in which, however, their move- 
ment is little restrained by the earth's magnetism. 
The coil is wound as shown in order that the 




Fig. 7. 






current may tend to turn each needle in .he same 
direction. 



Compensating Magnet. A compensating magnet 
is used with certain forms of galvanometers to 
neutralize the effect of the earth's magnetism on 
the needle. 



GALVANOMETERS. 11 

It is generally of a curved form as in Fig. 7 at 
M f being mounted on a rod upon which it can be 
slid up or down over the galvanometer G. It can 
also be rotated on its axis. 

An ordinary bar magnet is used for the same pur- 
pose by placing it in different positions w T ith rela- 
tion to the galvanometer until the desired result 
is obtained. 

Where the latter method is pursued it is a good 
plan to attach the magnet to a block of lead so 
that a slight jar will not displace it. 



CHAPTER II. 

Galvanometers. 

In describing galvanometers, the terms constant, 
figure of merit and sensibility are used. 

A full description of these terms rightly belongs 
to the pages on practical testing. 

But it may be stated here, that these terms are 
those by which galvanometers are compared when 
considering their sensitiveness. 

The constant or sensibility is the number of 
ohms resistance through which an e.m.f. of one 
volt will cause the galvanometer to give a deflec- 
tion of one degree on a standard scale. 

A more complete explanation of the terms will 
be found in a later chapter. 

The Thompson Reflecting Galvanometer. In the 

Thompson reflecting galvanometer, a flat mirror of 
less than one-half inch in diameter has a number 
of fine steel magnets fastened on its back. This 
mirror-magnet combination is suspended by a 
fibre of cocoon silk inside a coil of many turns of 
fine insulated wire. 

12 



GALVANOMETERS. 



13 



When the magnets are deflected by current flow- 
ing in the coil, the mirror also turns, and a beam 
of light directed on it is reflected on a curved scale. 




This scale has the zero mark in the centre, readings 
being thus possible in either direction. 



14 ELECTRICAL INSTRUMENTS. 

A compensating magnet is attached on a rod 
fixed to the top of the case and leveling screws are 
provided to level the instrument. 

As the mirror is small and light it has little 
momentum, its moving in a small chamber gives 
it an air cushion, and it quickly comes to rest or 
is " dead beat." 

In Fig. 8 is the tripod form of Thompson re- 
flecting galvanometer. 

This instrument is made with interchangeable 
coils from 150 ohms to 5000 ohms in resistance. 
The sensibility for minute currents with the 5000 
ohm coil is very great. 

A four coil reflecting galvanometer of the Thomp- 
son pattern in Fig. 9 has a hinged door whic 
opens, breaking the circuit through the coils 

This gives easy access to the suspension system 
Spring contacts C seen at the lower part of the 
base make connection at P when the door is shut. 

The magnetic system is suspended from a tube 
B equipped with an adjusting screw K . 

A compensating or control magnet M is fitted 
in a tube and adjusted by the milled head 5. 

Moving Coil Galvanometers. One of the greatest 
drawbacks to the use of galvanometers built on 
the Thompson pattern is their sensitiveness to 
external influences. In large cities where trolley 
cars and buildings of iron frame construction 
abound, this type of instrument needs constant 
adjustment. Allowances and checks have to 



)- 

; 



GALVANOMETERS. 



15 



enter into tests made with them so largely as to 
almost prohibit their use. 

A class of galvanometers based upon the rota- 
tion of a coil cairying current in a magnetic field 

«p K 




Fig. 9. 



has been so perfected that they are being almost 
universally adopted. 

The pioneer moving coil galvanometer is that 
invented by D'Arsonval. 



16 



ELECTRICAL INSTRUMENTS. 



The D'Arsonval Galvanometer. In Fig. 10 H is 
a horseshoe shaped magnet or series of magnets 
bolted together and to the base B which is sup- 
plied with leveling screws LL. 

A core of soft iron / is held stationary, around 



e 








- 



e 



e 



S 



Fig. 10. 

which turns a coil of fine insulated wire C. This 
coil is suspended between adjustable screws 5^4 
by fine wires in such manner that current entering 
at the screw 5 passes through the coil and out at 
A, or vice versa. 



GALVANOMETERS. 17 

A tiny mirror M of circular form is also carried 
by the suspension wires and turns with the coil. 

When current is applied to 5 and ,4 , the coil 
tends to turn in the magnetic field formed by the 
magnet poles and strengthened by the soft iron 
core /. 

The instrument is so constructed that the mag- 
netic field is as nearly uniform as possible, and 
the deflections of M are proportional to the current 
flowing through C. 

Horizontal Magnet. This type of D'Arsonval 
galvanometer is shown in elevation in Fig. 11. 

In this galvanometer the coil and mirror are 
suspended in a tube by a phosphor bronze strip. 

This form of construction permits the mirror 
and coil to swing but to return to zero without 
any distortion from twisting of the suspension or 
" set." 

Interchangeable tubes are furnished with coils 
of varying resistances and sensibilities. The 20 
ohm tube has a sensibility of 75 megohms (75 mil- 
lion ohms). The 4000 ohm tube has the extreme 
sensibility of 1750 megohms. 

The deflections in these instruments are propor- 
tional to the current. 

The moving system is either dead beat or bal- 
listic. 

The ballistic type is adapted for the measure- 
ment of currents of short duration. The current 
has ceased before the deflection is complete. But 



18 ELECTRICAL INSTRUMENTS. 

the impetus received by the coil carries it to a dis- 
tance and gives the possibility of measuring the 
current strength from the distance of swing. 

A form of D'Arsonval galvanometer, Fig. 12, is 
adapted for attachment to a wall. The galvano- 




Fig. 11. 

meter is entirely enclosed in a cylindrical case; the 
mirror deflections being observed through a win- 
dow in front. 

A removable bracket carries a reading telescope 
and scale so proportioned that it gives the same 



GALVANOMETERS. 



19 



effect as a standard millimeter scale at a distance 
of one meter. 

Gimbal suspension of the galvanometer itself 
causes it to be self-leveling. 




Fig. 12. 



The scale deflections are directly proportional 
to the current causing them. 

The sensibility is about 500 megohms per volt 
of battery used, a sensibility sufficient for all or- 
dinary measurements. 



20 



ELECTRICAL INSTRUMENTS. 



In the Willy oung form of D' Arson val galvano- 
meter the suspension of the coil is shown in Fig. 13. 

5 is the coil; U and L the suspensions, the latter 
being in the form of a spiral spring. T is a torsion 
head for adjustment of the coil to zero reading. 






Fig. 13. 



To clamp the coil for purposes of traveling, the 
head C is turned, E then raises the coil taking its 
weight from u and clamps it against D. 

The upper spindle A admits of the adjustment 



GALVANOMETERS. 21 

of new suspension wires. It is clamped by a set 
screw at e. 

The upper suspension is grounded on the tube 
case, the lower one L is connected to a platinum 
faced disc F. 

The electrical connections are therefore from 
one binding post through the case, down », coil L 




Fig. 14. 

and out through F to a spring connected to a 
binding post. 

A portable D'Arsonval galvanometer, Fig. 14, is 
constructed with a sensibility of upwards of one 
megohm. It is dead beat and has a scale divided 
from a central zero into 50 divisions right or left. 

As has been remarked before, instruments con- 
structed on the D'Arsonval principle read in direct 
proportion to the current passing. 



22 



ELECTRICAL INSTRUMENTS. 



And they are not affected by external magnetic 
fields. An instrument similar to the above with 
100 cells of battery would be capable of measuring 
insulations up to 100 megohms, and higher if frac- 
tions of a deflection be read. 




Fig. 15. 



Scale and Lamp. The oscillations of the gal- 
vanometer mirror are read in two ways. By 
watching a spot of light on a scale or observing 
the image of the scale reflected in the mirror by 
means of a telescope. 

The scale is made in various forms but in prin- 
ciple resembles that in Fig. 15. 



CHAPTER III. 

Rheostats, Keys and Shunts. 

Rheostats. The resistances used in testing-rheo- 
stats are mostly made of insulated wire having a 
high specific resistance, and but little liable to 
change under varying temperature conditions. 

They are also wound non-inductively so that 
no currents due to inductance shall affect the gal- 
vanometer. This end is obtained by winding the 
wire so that the current shall flow half around the 
spool in one direction and half around it in the 
reverse direction. 

The piece of wire may be either doubled upon 
itself and then wound on, or may be wound on in 
two equal lengths, the inside ends being soldered 
together. The outside ends are for attachment to 
the circuit. 

It is always desirable that the binding posts or 
terminals of a resistance coil be made large enough 
that they do not offer any perceptible resistance 
to the circuit. 

In the practical construction of rheostats for 
testing, they are adjustable in several ways. The 

23 



24 



ELECTRICAL INSTRUMENTS. 



cutting in or out of coils is effected by plugs or 
sliding switches. 

In the plug form of switch, Fig. 16, a brass taper 
plug P is inserted in a hole between brass strips 
5, connecting these strips together. 

The resistance coils R are connected as shown. 
If one plug be now removed from the hole, a coil 
becomes part of the circuit between A B. A sec- 
ond plug cuts in another coil and so on. Each coil 
thus cut in adds its resistance to the circuit. 





A B 



Fig. 16. 

The chief disadvantages of this system are that 
it requires a plug for each coil, and that if a plug 
be not tight a coil is not entirely cut out. 

Plug switches are also arranged to connect in 
coils when inserted in holes. 

Modern rheostats are being constructed, how- 
ever, with sliding switches which cannot get lost 
and which make good firm contact. 



Resistance Wires. The choice of a metal for 
resistances to be used for testing purposes must be 



RHEOSTATS, KEYS AND SHUNTS. 25 

determined by several points. The metal must 
change in resistance as little as possible under vary- 
ing temperatures. 

It is also desirable that the metal used shall have 
a high specific resistance, that is, its resistance per 
unit of length shall be as great as possible in com- 
parison with other metals. This is in order to 
keep the coil as small as possible, resistance coils 
with figures as high as 100,000 ohms being in com- 
mon use. 

German silver, although formerly much used, 
has been replaced by several compound metals. 

The composition of some commonly used resist- 
ance wires is as follows: German silver, copper 60 
parts, zinc 26 parts, nickel 14 parts. This alloy 
is also made with 30 per cent, nickel. 

Platinum silver contains platinum 67 parts, sil- 
ver 33 parts. 

Platinoid is German silver 98 parts, tungsten 2 
parts. 

Manganin contains copper S4 parts, manganese 
4 parts, nickel 12 parts. 

The relative resistances of the above are in com- 
parison with copper as 1, as follows: German silver 
12, platinum silver 15, platinoid 20 and man- 
ganin 30. 

As the proportions of these alloys is varied 
by different manufacturers, the figures given 
are but approximate. For example, if the plati- 
num silver alloy w r ere platinum 50 and silver 
50 parts, its relative resistance would be about 20 



26 ELECTRICAL INSTRUMENTS. 

as compared with copper, or more nearly that of 
platinoid. 

Of the above alloys, manganin seems to be the 
most favored by manufacturers of high-class in- 
struments, although platinoid is preferred by some. 

Glass Slab Resistances. It often becomes neces- 
sary to use a high resistance for testing purposes 
when none is at hand. A ground glass slab cov- 
ered with lead pencil lines will serve for temporary 
work. 

To make one proceed as follows: 

Take a slab of ground glass about 1 inch by 4 
inches and drill a hole at each end large enough to 
receive the machine screws at the bases of two 
binding posts. 

Rub a soft lead pencil over the slab around these 
holes covering the area of a circle of one inch or 
thereabouts. Blow off the dust and lay a few strips 
of tin-foil over the holes nearly covering the lead 
pencil c'rcles. Slip a washer over one machine 
screw and also a few wa hers of tin-foil and insert 
the screw in the hole from the plain side of the 
glass. The screws should penetrate the tin-foil 
on top of the glass also. Then screw down the 
binding post. 

The tin-foil below the slab acts as a cushion be- 
tween the washer and the glass. That on top of 
the slab also acts as a cushion, but it ensures a bet- 
ter contact between the binding post and pencil 
circle. 



RHEOSTATS, KEYS AND SHUNTS. 27 

A similar binding post is to be fitted at each end 
of the slab. 

Now draw a few lines joining the pencil circles. 
Measure the resistance of this penciled path. To 
lower its resistance rub on more pencil lead be- 
tween the circles. To raise its resistance, rub out 
some of the lead. Of course all dust should be 
blown off. 

The object of the circles was to give a point of 
junction between the lines and binding posts. 

Resistance slabs as above may be varnished or 
covered with insulating compound if desired. But 
pencil lead or plumbago being carbon is extremely 
erratic in the matter of maintaining a given re- 
sistance. 

Galvanometer Shunts. As in most cases it is not 
desirable to permit the entire current used in a 
test to flow through the galvanometer, part of it 
is shunted or caused to pass around the latter. 

A shunt bears a definite ratio to the resistance 
of the galvanometer, being generally adjustable to 

r h or ok* of lts resistance sothat to- m> 

or „ part of the current only passes into the 

galvanometer. 

Fig. 17 shows the connections of the shunts 5 
and galvanometer G. 

The decree in which the shunt increases the 



28 ELECTRICAL INSTRUMENTS. 

range of deflection of a galvanometer is termed 
its multiplying power. 

If one-tenth of the current flowing went through 
the galvanometer and nine-tenths through the 
shunt, the current in the circuit would be actually 
ten times that through the galvanometer. 

The current therefore in the galvanometer must 
be multiplied by the multiplying power of the 
shunt to show its true value in the circuit. 



r WW' 

I 
I 
I 



Fig. 17. 

In order to find the resistance necessary in a 
shunt to be used with a certain galvanometer, the 
resistance of the latter is to be divided by the 
multiplying power desired less one. 

As an example, let a shunt be needed for a gal- 
vanometer of 2000 ohms resistance where only one- 
fifth the total current is to pass through the gal- 
vanometer. This would equal a multiplying 

power of 5; then = 500 ohms. 

o _ 1 



RHEOSTATS, KEYS AND SHUNTS. 29 

Formula. Let G be resistance of galvanometer ; 
n be multiplying power of shunt; 5 resistance of 

Q 

shunt ; then 5 = — 

n-l. 

To find the multiplying power of a shunt of 
given resistance add its resistance to that of the 
galvanometer and divide the answer by the re- 
sistance of the shunt. 

For example, galvanometer is 10,000 ohms, 
shunt 1000 ohms, 10,000+1000 = 11,000, divided 
by 1000 = 11, the multiplying power of the shunt. 

Formula. Let resistance of galvanometer be G 

G -\- S 
and shunt 5; then — = — ■ = multiplying power. 

Shunts should be connected to the galvanometer 
by wires of ample size, no undue resistance should 
be introduced by the connecting wires. 

It is a good plan always to use the shunt of 
greatest multiplying pow T er at first and reduce as 
occasion requires. Otherwise a heavy current in 
the galvanometer might injure suspension. 

Condensers. The modern condenser is but a 
handy form of the Ley den jar; it consists of leaves 
of tin-foil alternating with insulated paper, mica, 
glass, etc., insulation between the tin-foil layers 
being of prime importance. 

The layers are built up as follows, Fig. 18: First 
a sheet of insulation A (or dielectric) , then a sheet 
of tin-foil B projecting at one end; another sheet 



30 ELECTRICAL INSTRUMENTS. 

of insulation C and the next sheet of tin-foil D 
projecting at the other end. 

Sheet after sheet is built up, Fig. 19, until the 
desired number is obtained, the whole mass then 
being subjected to immersion in paraffin or other 
insulator and kept under heavy pressure until set. 

The portions of tin-foil projecting from one end 
being pressed together form one connection, the 
ends at the other being the second connection. 

This results in every other sheet of tin-foil being 
connected, in faet the condenser can be considered 



Fig. 18. Pig. 10. 

as being made up of two large sheets of tin-foil 
highly insulated from each other. 

Condensers are made up with many different 
dielectrics, mica being one of the best for testing 
purposes. Tin-foil is most always used for the 
metal plates. 

Adjustable condensers permit of their capacities 
being varied by means of plugs which cut in or 
out of multiple, portions of the foil sheets. In 
Fig. 20 is the diagram of an adjustable condenser. 

C C are the tin-foil sheets, B B brass strips 
which carry the terminals or binding posts. 



RHEOSTATS, KEYS AND SHUNTS. 



S 



? ]& -, 



- G 



CD 



Fig. 20. 



32 ELECTRICAL INSTRUMENTS. 

The tin-foil sheets are connected to brass blocks 
G G in such proportion of the total number in the 
condenser as desired. 

By inserting taper brass plugs in holes between 
the brass strips and blocks as at P various com- 
binations may be made. 

A standard adjustable condenser is arranged so 
that the pairs of foil sheets may be connected in 
multiple as above, or in series. 

Keys. In electrical testing various kinds of 
switches or keys are used ; for opening or closing a 
circuit, for short circuiting, for ground connection 
and for reversing the direction of current flow. 

The insulated parts are made of hard rubber, 
contacts being equipped with platinum. 

In order to ensure the greatest possible insula- 
tion, the hard rubber pillars are often encircled 
by grooves. 

This decreases the leakage from dampness as it 
increases the distance between the conductor sup- 
ported and the table, measuring up the surface of 
the pillar. 

Reversing Key. A reversing key, Fig. 21, is 
used to reverse the direction of current flowing 
into the galvanometer. Two spring brass levers 
A B provided with hard rubber knobs are con- 
nected by means of binding posts to the galvano- 
meter G. A brass strip carrying platinum con- 



RHEOSTATS, KEYS AND SHUNTS. 



33 



tacts bridges the levers and a similar brass strip 
U passes under them. 

The battery is connected as shown, one ter- 
minal to the lower strip and the other terminal 
to the upper strip. 

When both A B are up they make contact with 
0, and form a short circuit to the galvanometer 
G, and likewise when both levers are held down 
on strip U. 



-J2 


IIIII-- 


""! + 

1 










( 





Lel 




c 


o) 






u 








Co) 






(°) 




T 

_ j 

Fig. 21. 















— X- 




But if one lever ^4, for example, be pressed down 
the negative terminal of the battery is connected 
to A and the positive terminal to B and thence 
to the galvanometer. 

On the other hand, if B only is pressed down 
the current to G is reversed because B is now 
connected to the negative terminal of the battery 
and A to the positive. 

This key should not be so connected that the 



34 



ELECTRICAL INSTRUMENTS. 



platinum contacts form part of a circuit, the re- 
sistance of which is being measured. The contact 
resistance will vary as it is not possible to hold a 
spring key down with an unvarying pressure. 

It will be seen that under no circumstances 
should the battery be connected to the terminals 




Fig. 22. 



at the end of A and B, a short circuit of the bat- 
tery would result. 

The Rymer Jones reversing key shown in Fig. 
22 makes a rubbing or wiping contact. The levers 
moved by the hard rubber handles are equipped 
with large contacts of platinum. 



RHEOSTATS, KEYS AND SHUNTS. 



35 



The contacts on which these levers work are 
also faced with platinum. 

A rubbing contact of this nature ensures but a 
nominal resistance. In fact, it can be neglected 
in ordinary operations. 

Keys of this style of construction are always 
preferable to those with striking contacts. 




Fig. 23. 



Commutator. A form of reversing switch or 
commutator is shown in Fig. 23. By changing 
the plugs into different holes, circuits connected to 
the commutator may be reversed, put to earth, or 
short circuited. 



36 



ELECTRICAL INSTRUMENTS. 



The brass pieces are mounted on hard rubber 
pillars to increase the insulating distance from each 
other and from the base. 



Discharge Key. A discharge key of the Kempe 
pattern, Fig. 24, has two triggers controlled by 
buttons. One button is marked " insulate," the 
other " discharge." 




Fig. l>J. 

There are three binding posts, one connected to 

a movable lever operated by a button, one to a 
contact underneath this lever, and one to a contact 
over this lever. 

When the lever is up, it presses against the upper 
contact completing a circuit connected to the 
lever binding post and top contact binding post. 

When the lever is down, it presses against the 
lower contact; the circuit is now through the 
lever and lower contacts and their respective bind- 
ing posts. 



RHEOSTATS, KEYS AND SHUNTS. 37 

But in certain tests it becomes necessary to 
leave open the circuit connected to the lever. 

The key provides for this in its second operation 
as follows: 

Depress lever all the way down and it locks 
under a trigger controlled by the " insulate " but- 
ton. Lower contact and lever are together. Press 
" insulate " button and lever flies up. But not all 
the way, it is caught and held in mid-air by the 
trigger attached to the " discharge " button. The 
circuit attached to the lever is thus broken. 

Third, press " discharge " button and lever 
being released rises and rests against top contact. 

The Standard Cell. A standard cell is a battery 
cell used in testing which maintains a steady e.m.f. 
of known value. It is used principally for com- 
parison of other cells and to check voltmeters. 

A standard cell is not required to give a large 
current, in actual work the current flow is kept 
as low as possible. 

A standard cell has a thermometer included in 
the case as a small correction must be made for 
changes of temperature. 

For tests not requiring extreme accuracy, the 
Daniel or copper sulphate cell may be used. Its 
e.m.f. is so near one volt as to be usable as such. 
A number of Daniel cells carefully set up and con- 
nected in series may be measured as to e.m.f. and 
the reading used for tests. 

But for accurate work the Clark or the Weston 
cell is necessary. 



38 



ELECTRICAL INSTRUMENTS. 



The Clark cell has a positive element of mercury 
and a negative element of zinc sulphate and rner- 
curous sulphate. Platinum electrodes form the 
connections in these elements. The e.m.f. at 15° 
C. is 1.434 volts. 




Fig. 25, 



The Weston cell, Fig. 25, has a positive element 
of mercury also, but the negative element is cad- 
mium amalgam in a saturated solution of cadmium 
sulphate. Its e.m.f. is 1.019 volts. 

The actual construction of standard cells may 
be found in any of the large works on testing. 



CHAPTER IV. 

Voltmeters and Ammeters. 

The Voltmeter. A voltmeter is merely a gal- 
vanometer of high resistance connected across two 
conductors of opposite polarity. 

The resistance of the voltmeter is extremely 
high in comparison with that of the conducuors, 
and but a minute current flows through it. 

As this resistance is fixed the only way to vary 
the amount of current flow is to vary the e.m.f. 

An increased e.m.f. will increase the current flow 
and likewise a decrease of one will produce a de- 
crease of the other. 

And these current variations producing corre- 
sponding deflections of the needle, the deflections 
show actually the changes of e.m.f. 

The scale divisions are calibrated or given a 
value in volts either by using e.m.fs. of known 
value, or by means of a standard voltmeter. 

Voltmeters are connected across the circuit as 
in Fig. 26. 

The Series Ammeter. There are two classes of 
ammeters, the series and the shunt. 

39 



40 



ELECTRICAL INSTRUMENTS. 



In the former the entire anient to be measured 
passes through the coils of the instrument, Fig.* 27, 
and its changes of value directly affect the needle 
as in the simple galvanometer. Changes of e.m.f. 
will therefore not affect the ammeter directly but 
only' in that they vary the current flowing in the 
circuit in which the ammeter is connected. 

Series ammeters are unwieldy for large currents 
as will readily be seen, for the coils must be large 
enough to carry all the current of the circuit. 
They are therefore almost entirely displaced by 
the shunt ammeters. 



rr 



u 



ic a 



Fig. 20. 



The Shunt Ammeter. The shunt ammeter is 
really a voltmeter calibrated to read in amperes 
and dependent upon the changes of e.m.f. in a 
portion of the circuit. 

The current in a circuit of given resistance is 
controlled by the e.m.f. at its terminals. 

Ohms law teaches that the current or 7 equals 
the e.m.f. or E divided by the resistance or R. as 



a formula / = — 
R 



VOLTMETERS AND AMMETERS. 



41 



If then there is an increased current flow in a 
circuit it must be due to an increased e.m.f . unless 
the resistance is changed. 

By taking a portion of the circuit and connecting 
a voltmeter across it, the variations of e.m.f. caus- 
ing the current flow will be indicated. 

In the shunt ammeter, Fig. 28, a German silver 
shunt 5 or one made of a special alloy is connected 
in series with the main circuit A B. 

A low reading voltmeter A is connected to the 
opposite ends of the shunt. 





Fig. 27. 



Fig. 28. 



Variations of e.m.f. across the terminals of 5 
will affect A, which is calibrated in amperes by 
comparison with a standard or by calculation. 



The Weston Voltmeter. The principle of the 
Weston voltmeter or ammeter for direct current 
is that of the D'Arsonval galvanometer. The mov- 
ing coil is mounted on pivots between jeweled cen- 
tres instead of being suspended from wires or 
threads. This mounting permits of portability 
and compactness. 



42 



ELECTRICAL INSTRUMENTS. 



The construction of the moving system and 
magnet pole pieces is shown in Fig. 29. 

The portable type of Weston voltmeter and 
ammeter is shown in Fie:. 30. 




Fig. 29. 



In the portable instruments the data of resist- 
ance, etc., will be found printed on the inside of 
the lid. Care should be taken not to interchange 



VOLTMETERS AND AMMETERS, 



43 



the case lids, each instrument bears a serial num- 
ber. 

The laboratory type, Fig. 31, has a very large 
scale of peculiar design, Fig. 32, which admits of 
readings being made to a fraction of one degree of 
deflection. 

Types of Weston switchboard instruments are 
shown in Figs. 33 and 34, the latter being made 
to fit flush with the face of a switchboard. 




Fig. 30. 



Figs. 35 and 36 show forms which have trans- 
parent dials illuminated from behind. 

Instruments of this pattern are often mounted 
on a swinging arm that they may be read from 
various parts of the room. 

A black disc controlled by a button in the middle 
of the case front, is set to a determined point. 



44 



ELECTRICAL INSTRUMENTS. 




VOLTMETERS AND AMMETERS. 



45 



The pointer is then more readily located as to its 
proximity to this disc when viewed from a distance. 

Sensibility. In the Weston direct current in- 
struments of the portable or the Type B form, a 
resistance of about 100 ohms per volt is added. 

In the other types of switchboard voltmeters- 
the resistance is usually about 65 ohms per volt. 




Fig. 33. 



In case the exact resistance is not marked, it 
can be obtained from the makers by quoting the 
serial number of the particular voltmeter. 

As the figures are needed in testing work, they 
should always be recorded. 

The sensibility of a Weston instrument type B or 
of the portable form is about 10,000 ohms. One 



46 



ELECTRICAL INSTRUMENTS. 



scale division of deflection represents one . volt 
through 10,000 ohms if 100 divisions equal one 
volt through 100 ohms. 

The current used will equal the voltage indi- 
cated divided by the resistance of the instrument. 
For example, in a 150 volt voltmeter of 15,000 

ohms, TeTwT = ampere for full scale deflec- 




Fig. 34. 



tion. One scale division hare representing one 



volt would equal 



15000 



of an ampere. 



Switchboard Type. The energy necessary to 
operate the Weston switchboard type does not 
exceed .05 of one per cent, of the total energy being 
measured. 



VOLTMETERS AND AMMETERS. 



47 




Fig. 35. 




Fig. 36. 



48 ELECTRICAL INSTRUMENTS. 

As an example of close reading, a change of 1 
ampere may be detected on an instrument reading 
to 1200 amperes at any point in the scale. 

Most switchboard instruments are calibrated for 
a temperature of 90° F., the error amounts to one 
per cent, for a change of 10° F. above or below 90°. 

The resistance of the coils will be increased by a 




Fig. 37. 

rise and decreased by a fall of temperature. The 
instrument therefore reads lower for an increase 
of temperature than the actual e.m.f. and vice versa. 

Duplex Instruments. Most of these instruments, 
Fig. 37, consist of a voltmeter and an ammeter 
combined in one case. 



VOLTMETERS AND AMMETERS. 49 

But if desired two voltmeters or two ammeters 
may be so combined. 

They are most suitable for automobile and motor 
switchboard w r ork, being constructed to stand a 
maximum of vibration. 

In connecting up automobile instruments where 
the wires run under the mat, some precaution 
must be taken against injury to the wire. The 
customary slipshod manner of pulling a flexible 
wire beneath the mat where it is forever being 
abraded, is to be condemned. 

No instrument will indicate correctly with poor 
connections and this is particularly true of shunt 
ammeters. 

Any change in resistance of the wires between 
shunt and instrument will affect the readings. 

Potential Indicators. These instruments are so 
constructed as to give large indications for slight 
changes of current or e.m.f. 

In order to keep the scale within limits, the poin- 
ter does not move until the e.m.f. (for example) is 
near the average. In Fig. 38 is an illustrative scale 
for an e.m.f. of 500 volts. 

The index or pointer commences to move as 
soon as the e.m.f. increases above 400, reading 
e.m.f. on lower scale. 

The upper scale with central zero shows the num- 
ber of volts above or below 500 as it will be seen 
that adding the upper left scale reading and that 
on the lower left scale directly underneath it, the 



50 



ELECTRICAL INSTRUMENTS. 



total will be 500. And subtracting the upper .read- 
ing from the lower reading on the right scale also 
gives 500. 

Differential Voltmeters. Differential voltmeters 
have a central zero and are used in connection 
with two independent sources of e.m.f. 

In the case of two generators which are to be 
run in parallel, the generators are adjusted until 
the needle stands at zero, when the e.m.fs. of both 




Fig. 38 



are equal. The needle points towards the side of 
higher e.m.f. 

When the two generators are being operated, the 
readings on either side must not be taken to rep- 
resent the e.m.f. of that particular generator. 
The readings indicate the difference between the 
generators. 



Double Scale Instruments. Indicating instru- 
ments are provided with double scales so that a 
greater range of measurement is possible. 



VOLTMETERS AND AMMETERS. 51 

For example, the upper scale might read from 
to 150 volts and the lower scale from to 15 
volts. In this case as the instruments are all 
direct reading with deflections proportional to the 
current, the resistance put in circuit for the higher 
reading would be ten times that for the lower. 

There are generally two marked binding posts 
on one side and one on the other. Care must be 
taken to connect to the correct posts. In an in- 
strument at hand, the same scale reading to 6 volts 
has three readings. The 6 volt total scale has 
521.6 ohms in circuit with the coil, the 60 volt = 
5215 ohms and the 240 volt = 20869 ohms. 

A simple calculation will show that all these re- 
sistances are almost exactly proportional to the 
total scale readings. 

Multipliers. Multipliers for increasing the read- 
ings of voltmeters are largely used. They are re- 
sistance coils in portable cases, Fig. 39, and are 
put in series with the voltmeter. 

Multipliers must be adjusted for each particular 
instrument as the resistance coil must be a mul- 
tiple of the voltmeter resistance. A multiplier 
with a value of 10, for instance, used with a 6 volt 
voltmeter of 521 ohms would measure about 5215 
ohms; one with a value of 40 w r ould equal about 
20,860 ohms. The multiplier by 10 would give a 
total scale value of 60 and the multiplier by 40, a 
total scale value of 240 volts to the 6 volt instru- 
ment. 



52 



ELECTRICAL INSTRUMENTS. 



The value of such an apparatus in practical 
work is very great. It does away with the neces- 
sity of having a number of voltmeters of different 
ranges. 




Fig. 39. 



Millivolt and Milliampere Instruments. The 

prefix milli means one thousandth, a millivolt is 
an e.m.f. of one thousandth of a volt, a milam- 
pere or milliampere, one thousandth of an ampere. 
Instruments of which the total scale reading is 
about one volt or one ampere, and those in which 
each scale division represents one thousandth of 
a unit, are millivoltmeters or milammeters respec- 
tivelv. 



VOLTMETERS AND AMMETERS. 



53 



Such instruments are of the utmost service in 
measuring low resistances, temperature changes, 




Fig. 40. 



etc., and their use will be described in the chapter 
on testing. 



54 ELECTRICAL INSTRUMENTS. 

Relative Sizes. Fig. 40 embodies the relative 
sizes and styles of Weston instruments with their 
style letters. 

Hot Wire Instruments. Voltmeters and am- 
meters indicating from the expansion and contrac- 
tion of wires carrying current were first repre- 
sented by the Cardew types. 

A fine wire was run over a pulley carrying an 
index, one end of the wire being rigidly attached 
and the other end held taut by a spring. When 
current passed along this wire, the latter expanded, 
the slack was taken up by the spring and the 
pulley rotated. Upon cessation of the current, 
the wire cooled and contracted, the pulley reversed 
and its pointer returned to zero. 

Instruments made on this principle could be used 
on either direct or alternating currents. 

But they possessed the disadvantages of burning 
out on accidental overloads, of the wire becoming 
stretched or otherwise distorted, of consuming a 
larger current than the other types which would 
render them less accurate, of requiring a long case 
and an undesirable position of that case and many 
minor defects. 

The Whitney Hot Wire Instruments. The Whit- 
ney type of hot wire instruments represents the 
practical solution of the before mentioned defects. 

The mechanism is shown in diagram in Fig. 41. 

A wire A B of high resistance, low temperature 



VOLTMETERS AND AMMETERS. 



55 



coefficient and non-oxidizable metal is secured at 
one end to a plate C. It runs over a pulley D 
mounted on a shaft E, and its free end is attached to 
the other end of C, but is insulated therefrom. 
A spring F keeps the wire taut and takes up any- 
slack imparted to the wire by the passage of cur- 
rent. 




Fig. 41. 



Current only flows in the portion of w r ire .4 be- 
tween C and the pulley D where the coiled wire 
connection is shown. 

When A is heated by current flow it expands, 
F taking up the slack, pulls ^4 round D y and ro- 
tates D and E. 



56 



ELECTRICAL INSTRUMENTS. 



If a pointer were directly attached to shaft E 
it would indicate on a scale the movement of D 
and E. 

But a magnifying device is employed. G is a 
forked rod rigidly attached to E, at its lower end 
a silk fibre fastened between the forks passes around 
a pulley H which carries the pointer. 




Fig. 42. 



The rotating of E therefore moves G and the 
silk fibre rotates H and the pointer, thus magni- 
fying the movement of G. 

Any temperature variations will affect A B 
equally, it is therefore self-compensating. 



VOLTMETERS AND AMMETERS. 



57 



The tension of F being slight, wire A B is not 
under a heavy strain, it more nearly returns to 
its original length after current ceases. 

The instrument, however, can be adjusted to 
zero very readily if it should become necesssary. 

A portable instrument is shown in Fig. 42 and 




Fig. 43. 



a switchboard instrument in Fig. 43, many other 
types being manufactured. 

For direct current work only an electro-magnetic 
system is used. The ammeters of this type have 
a uniform drop of .05 volt. Shunts are thus inter- 
changeable. 



58 



ELECTRICAL INSTRUMENTS. 



Ammeter Shunts. The ammeter shunts used 
with Weston switchboard instruments are shown 
in Fig. 44. 

The shunt itself is made of one or more sheets 
of alloy resembling German silver in appearance. 
The ends of these sheets are fastened into brass 
or copper blocks. 




Fig. 44. 



Large clamping screws are provided for attach- 
ment of the shunt to the bus-bar, which latter are 
inserted in the large slots at each end of the shunt. 
The small screws are for the instrument cords. 

The Whitney shunt, Fig. 45, is made of a solid 
block of metal slotted as shown so as to give the 
resistance of a long bar. 

In all cases the connections to shunts must be 



VOLTMETERS AXD AMMETERS. 59 

as perfect as possible. Poor contact causes heat- 
ing if at the bus-bar connections and thereby in- 
correct indicating at the instrument terminals. 

The shunt cords must also be properly clamped 
cr incorrect readings will result. 

The importance of these precautions will be 
thoroughly appreciated when the principle of the 
shunt ammeter is considered. 

For the same reason the cords must on no account 
be tampered with. If too long loop them up and 




Fig. 45. 

bind the looped part if desired with tape. A 
shortened cord would increase reading of instru- 
ments as it lowers the resistance in series with the 
ammeter. And a poor connection would decrease 
readings on account of its resistance. 

Too little attention is often given to these points 
by those in charge of a plant, an instrument can- 
not give correct readings if improperly connected. 

Shunts for portable instruments are included in 
the base of the instrument. 

For extreme adaptability, a millivoltmeter can 



60 ELECTRICAL INSTRUMENTS. 

be equipped with a set of shunts so that the same 
instrument will measure from a fraction of an 
ampere to many thousand amperes. 

Other Types of Instruments. Voltmeters and 
ammeters are constructed on principles other than 
that of the D'Arsonval galvanometer, or the 
stretching of a hot wire. 

It is unquestioned that the D'Arsonval system 
is the only correct one for instruments to be used 
on direct current circuits only. But owing to 
patent rights it has been necessary for manufac- 
turers to seek other methods of construction. 

The result is that while no new principles are 
used, those less suitable than the D'Arsonval 
have been perfected with such ingenuity as to 
make most excellent substitutes. 

It is also unquestioned that when the D'Arson- 
val patent expires it will be the only principle 
that will survive. That is, unless the electro- 
static instruments are much improved and made 
practicable for measurements of low potentials. 
The aoove refers more to laboratory and portable 
instruments than to those designed for switch- 
board work. 

The solenoid type of Fig. 46 has a pivoted soft 
iron armature curved to enter a solenoid. Current 
being passed through the wire of the solenoid 
causes the armature to be attracted more or less 
against a restraining force. The latter may be 
gravity or springs. 



VOLTMETERS AND AMMETERS. 



61 



A pointer attached to the armature indicates 
on a dial. 

A simpler solenoidal instrument is shown in Fig. 
47 but acts in the same manner. 

A type of magnetic vane instrument, Fig. 48, 
depends upon the repulsion between two pieces 
of iron in the same field. One is stationary inside 
a coil of wire, the other is movable on an axis. 

As both lie in the coil the same way they both 





Fig. 46. 



Fig. 47. 



Fig. 48. 



have N poles or 5 poles at the same end. As simi- 
lar magnetic poles repel each other the stronger 
these iron pieces become magnetized the farther 
apart they move. 

The galvanometer principle, Fig. 49, is used 
where a short magnet is pivoted inside a coil large 
enough to give a fairly uniform field. 

In Fig. 50 the attraction is between two curved 
armatures and the two poles of an electro magnet. 



62 ELECTRICAL INSTRUMENTS. 

This is purely magnetic attraction and is counter- 
balanced by the force of gravity, a spring, or an 
adjustable weight. 

The heating effects of current are used in Fig. 
51 where a coil of wire surrounds a thermometer 
bulb. By the rise of the mercury the heat and 
thereby the current flow is measured. 

Westinghouse Type K. The Westinghouse type 
K voltmeters and ammeters are made on the prin- 




Fig. 49. Fig. 50. Fig. 51. 

ciple illustrated in Fig. 47, the lower end of the 
armature working in a glass tube filled with oil. 
This steadies the moving system and renders it 
dead beat. The armature is not rigidly attached 
to the pointer shaft, but has a flexible connection 
of silk. The shaft itself takes the form of a scale 
beam working on knife edges. The bearings for 
the knife edges are so constructed that the leverage 



VOLTMETERS AND AMMETERS. 



63 



becomes uniform and the divisions remarkably 
even throughout the scale. 

Being purely electromagnetic attracting instru- 
ments they can be used on circuits of either direct 
or alternating current. 




Fig. 52. 



G. E. Potential Indicators. A simple form of 
voltmeter and also of ammeter made by the Gen- 
eral Electric Company is illustrated in Fig. 52. The 
movement of the armature in the solenoid is con- 
trolled by gravity, means of adjustment being by 
means of the balance weights shown in the illus- 



64 ELECTRICAL INSTRUMENTS. 

tration. The ammeters of this type are series 
connected not shunted. 

Thomson Inclined Coil Meter. The Thomson in- 
clined coil ammeter is adapted for use on circuits 
of alternating current. 

The instrument is constructed on the magnetic 
vane principle in which an iron vane or wing 
strives to turn itself parallel with the axis of a 
coil carrying the current to be measured. 




Fig. 53. 

The name comes from the peculiar position of 
the coil. Instead of lying flat on the base its 
axis is inclined. 

A soft iron vane is mounted obliquely on a 
shaft which latter is held in a vertical position 
and controlled by springs. 

The shaft also carries a pointer moving over a 
graduated dial. 



VOLTMETERS AND AMMETERS. 



65 



When current is sent through the coil the vane 
turns against the springs so as to set itself parallel 
with the axis of the coil. The inclined position 
of the latter and the peculiar shape of the vane 
give a more evenly divided scale than is obtained 
in most other instruments of this class. 




Fig. 54. 



The voltmeter has a similarly placed stationary 
coil, but in place of the iron vane is equipped 
with a moving coil in series with the other coil. 

The pointer is carried by the moving coil sys- 
tem which is controlled by springs. 



66 



ELECTRICAL INSTRUMENTS. 



This type of instrument can also be used on 
direct current circuits. 

On alternating current circuits they work 
equally well for all frequencies of alternations. 

The mechanism of the pocket ammeter is shown 




Fig. 55. 



in Fig. 53, the switchboard ammeter in Fig. 54, 
and the switchboard " edgewise M voltmeter with 
iron vane moving system in Fig. 55. The am- 
meters below 500 amperes total scale are series, 
the entire current passing through the instrument. 
Although the above type may be used on direct 



VOLTMETERS AND AMMETERS. 67 

current circuits, the manufacturers recommend 
the Thomson astatic form for such circuits. 

The Thomson Astatic Voltmeter. The mechan- 
ism is illustrated in Fig. 56, the same principle 
being used in the astatic ammeter which, how- 
ever, is a shunt instrument. 

The pointer shaft carries a coil through which 




Fig. 56. 

passes the current to be measured and also two 
pieces of magnetic metal. 

The field magnets are electromagnetic, not per- 
manent, as in the D'Arsonval instruments. 

An astatically arranged magnetic field is mount- 



C8 ELECTRICAL INSTRUMENTS. 

ed perpendicularly to the shaft and restrains the 
pieces mounted on the shaft. 

Deflections of the coil due to current flowing 
through it are opposed by the magnetic field act- 
ing on the pieces of metal which are carried by 
the same shaft. There are thus no restraining 
springs, current to the moving coil being con- 
veyed by torsionless spirals of silver wire. The 
illustration shows the illuminated dial voltmeter 
one of the miniature lamps for lighting the dial 
being visible. The resistance spools are also 
shown, two on each side of the electromagnet coils. 

The dead beat or damping effect is produced 
by an aluminum disc which moves in the field of 
an electromagnet. 

Thomson astatic instruments can be provided 
with polarity indicators, a red disc showing in 
the scale card when polarity is reversed. 

The Wattmeter. The most efficient forms of 
wattmeters are constructed on the electro-dynamo- 
meter principle. 

If two coils of wire are constructed, one sta- 
tionary and the other free to turn inside it, the 
movable one will tend to turn until its coils are 
parallel with the stationary one when current is 
applied to both. 

In the wattmeter, the stationary coil is of heavy 
wire and the main current flows through it. The 
movable coil is of fine wire in shunt across the 
circuit. 



VOLTMETERS AND AMMETERS. GO 

Changes of current strength in the main circuit 
thus affect one coil, and changes of e.m.f. the other. 

As the watt is the product of the e.m.f. multi- 
plied by the current, the index attached to the 
movable coil is influenced by the watts in the cir- 
cuit. 

This type of instrument can be used on direct 
or alternating current lines. 

Keystone Instruments. The Keystone voltmeters 
at present are made either on the electro-dynamom- 
eter principle just described, or are electromagnetic. 

In both types they can be calibrated to read 
on alternating or direct current circuits. 

The makers recommend the electro-dynamo- 
meter type for direct current voltmeters, but not 
for ammeters. The connections between the mov- 
ing coil and the circuit would have to be so large 
as to interpose friction. 

The ammeters are electromagnetic and are for 
series connection. The scales, however, are re- 
markably even. 

In the electro-dynamometer type, there are 
two springs to control the moving coil. 

Over the first part of the scale a spiral spring 
opposes the movement, while a " pendulum " 
spring assists it. Further on, the latter spring 
assists the spiral spring, thus opposing the coil 
movement when the deflecting force is greatest. 

This ensures greater evenness in the scale divi- 
sions throughout the range. 



70 ELECTRICAL INSTRUMENTS. 

Queen Instruments. The Queen Wirt voltmet- 
ers and ammeters can be used on circuits of 
either direct or alternating current. 

They are electromagnetic, the principle being 
that in which an iron armature tends to move 
into the strongest part of a magnetic field. 

A tube of soft iron lies parallel with the axis 
of a coil of wire through which current passes. 
Being mounted away from the true centre of the 
coil it moves towards that centre where the field 
is strongest. 

The electromagnetic principle does not allow of 
absolutely even scale divisions, but those in the 
middle of the scale are fairly uniform. 

Although for use on both circuits, the scales 
must be graduated for either one or the other. 

For alternating current the divisions are most 
uniform. 

In the Queen portable form, an upper scale can 
be graduated for direct current and a lower scale 
for alternating current. 

Electro-Static Instruments. A type of instru- 
ment much used on circuits of extremely high 
e.m.f. is made on the principle that two oppositely 
charged plates will attract each other. 

A movable plate of aluminum is suspended be- 
tween fixed plates of similar metal. The jnovable 
plate carries an index and is controlled either by 
gravity or springs, generally the former. 

The fixed plates are connected to one side of 



VOLTMETERS AND AMMETERS. 71 

the circuit and the movable plate to the other 
ride. 

The instrument in fact is a form of condenser. 

When a source of high e.m.f. is connected as 
above to the instrument the mutual attraction 
between the two series of plates causes the movable 
plate to swing and its index to indicate on a scale. 

As the attractive force depends upon the square 
of the e.m.f., the scale divisions are not uniform. 

Such instruments consume no current. 

The chief advantage of the electrostatic volt- 
meter is that it can be directly connected to a 
circuit of extremely high potential. 

In the case of alternating current circuits with 
voltages up in the tens of thousands it becomes 
necessary with other types of voltmeters to reduce 
the voltage before applying it to the instrument. 

This is done by means of a " potential transfor- 
mer, " which is a small transformer especially con- 
structed to reduce the high voltage to a lower 
one suitable to the instrument in use. 

The voltmeter is thus not directly connected to 
the circuit but is inductively connected. 

Reading Instruments — Parallax. One source of 
error in reading voltmeters and ammeters lies in 
the fact that the pointer or index does not touch 
the scale. 

If the eye is not directly in front of the index 
a reading to one side or the other is liable to be 
made. 



i'l ELECTRICAL INSTRUMENTS. 

Where an index stands as much as a quarter of 
an inch away from the scale, a large error in read- 
ing might result. 

This error is called parallax and is prevented 
in portable instruments for accurate testing by 
the following device: 

The index is made thin and flat, lying edgewise 
towards the eye. A strip of mirror is placed be- 




Fig. 



neath the index. The reading is made when by 
looking down on index and mirror, the index 
hides its own reflection. 

Some switchboard instruments are made for 
edgewise reading, that is, the scale is at right angles 
to the base, Fig. 57. The scale is curved and the 
index is bent so as to follow it. 



VOLTMETERS AND AMMETERS. 73 

In this type, the error of parallax is likely to be 
greater than with the ordinary type. 

Care of Instruments. Electrical measuring in- 
struments must be handled with at least ordinary 
care. 

Although made as air tight as possible, damp- 
ness and excessive heat should not be permitted 
to attack them. 

Violent swinging of a pointer through excessive 
current is liable to do more than bend the pointer. 

Even if it does not permanently injure the in- 
strument it does it no good. This is more likely 
to happen in using a double scale instrument, cr 
by reversing the connections. 

When taking measurements the instrument 
should not be set down on a generator or its bed 
plate. 

And a direct current voltmeter should not be 
tried on an alternating current circuit. 

The glass front should never be rubbed befoi'C 
taking a reading, or the pointer will be influenced 
by so-called static currents. 

They may be dissipated by touching the finger 
to the middle of the cover, but are best omitted. 

In the case of two or more compound wound 
generators being connected together to run in 
multiple, a so-called equalizer is used. This is a 
bus-bar or cable connecting together a point on 
each dynamo between the series field winding and 
its brush connection. 



74 ELECTRICAL INSTRUMENTS. 

The ammeter or its shunt is not to be connected 
in series with the lead from the series field or 
equalizer side, but in that lead which runs directly 
from the brush to the main switch or bus-bar. 
Otherwise current flowing in the equalizer bus 
would not be indicated. 

Ammeters for storage battery circuits are made 
with a central zero as the deflections of charge 
or discharge are in different directions. 

This is of course only true of that class of in- 
strument in which the direction of deflection de- 
pends upon the direction of current flow, that is, 
polarized instruments. 

If a polarized instrument without a central zero 
be used for storage battery work, a reversing 
switch becomes necessary between the shunt and 
the ammeter. 

This is but a makeshift and is by no means to 
be recommended. 

Any instrument built to be used only in a ver- 
tical position must not be used when laid hori- 
zontally and vice versa. Readings may be made, 
but they are liable to be inaccurate, owing to fric- 
tion or gravity. 



CHAPTER V. 
The Wheatstone Bridge. 

The Wheatstone Bridge. In Fig. 58 is shown the 
lozenge or kite diagram of the Wheatstone bridge. 

A B are two adjustable resistances. These form 
the proportional arms of the bridge. 

R is an adjustable resistance or rheostat, G a 
galvanometer, B the battery and K K keys closing 
the battery or galvanometer circuits respectively. 
x is a resistance which it is desired to measure. 

If the battery key be closed, current will divide 
between A and B and flow through R and x back 
to battery B. 

If A B, R and x are equal, the current will di- 
vide equally down each side of the lozenge. Even 
if the galvanometer key be closed, no current will 
flow into G. 

If x be greater in resistance than R, part of the 
current through B will travel through G and thence 
through R in accordance with the law of shunts. 
The galvanometer needle is thus deflected. But 
by adjusting R to agree with x the tw r o paths R 
and x being equal no current flows through G. 

Should R be greater than x, current flows through 

75 



76 



ELECTRICAL INSTRUMENTS. 



A and through x by way of the galvanometer, and 
the needle is deflected, but in the opposite direc- 
tion to the former case. 

The Whcatstone bridge is based on the fact that 
no current flows between points of equal potential. 

Consider the two sides of the bridge A R and 
B x as two wires of equal resistance per unit 
length in multiple between the battery terminals. 

The drop of e.m.f. will be the same in each wire 
for the same length. 




Fig. 58. 



If the galvanometer be connected to a point in 
each wire equally distant from either end, say the 
middle of each, no current will flow in the gal- 
vanometer. 

The effect is the same as if both terminals of 
the galvanometer were placed on the same spot 
in a single wire carrying current. 

Shift one galvanometer connection, this has the 



THE WHEATSTONE BRIDGE. 77 

same effect as separating the two contacts on a 
single wire. 

The galvanometer in the latter case will be in 
shunt with the piece of wire included between its 
contacts. As the points are at a different poten- 
tial, the galvanometer will show r the potential in 
that piece of the circuit. 

The same applies to the double wires. When 
the bridge galvanometer is connected as in the 
figure but assuming A R B and x as equal, the 
galvanometer is connected between tw T o points of 
the same potential. 

If x be increased in resistance over that of R it 
would be the same as increasing the length of the 
wire x. 

In the latter case the galvanometer contact 
could be shifted along the x wire until it again 
stood at the middle, when the galvanometer would 
remain undeflected. 

In the bridge the resistance R would be ad- 
justed until it was equal to x, A being in the same 
proportion to R as B was to x, the galvanometer 
would be at the points of equal potential. Of 
course B could be changed but R is the adjustable 
resistance. 

If A is to B as R is to x, then % will be the same 
as B multiplied by R and divided by .4. 

This is just simple proportion in arithmetic as the 
following example shows. In simple proportion, to 
get the fourth term or answer, the second and third 
are multiplied together and divided by the first. 



78 



ELECTRICAL INSTRUMENTS 



For example, if 1 = 2 what will 4 equal? 

9x4 

1:2 :: 4 : x or ^— - S. 

And in the foregoing formula, if 
A — B what will R equal? 

A : B : : R : *, or — -. — = x. 



S A | 2 o B S 

rhafEzr rzr s? izz izf 2 



.□IZJ 



—a 



—-—J 



1~LLU 
IZirZLI 



3 



z2_ 



fcf 



X -- 



QXD-CBKBKEKD- 




Fig. 59. 



The Post-Office Bridge. TTre connection for re- 
sistance tests of the form of bridge known as the 
(English) Post-Office Bridge is in Fig. 50. 

A tripod form of Thompson galvanometer is 
illustrated and an 18 cell battery but it is evident 
that a larger or smaller battery could be used. 



THE WHEATSTONE BRIDGE. 79 

And any form of galvanometer would be connected 
up in the same way. 

The shunt is not shown, it would be connected 
across the galvanometer terminals. The resist- 
ance to be measured is connected at x. 

The key on the left of the bridge is the battery 
key and should always be depressed first. The 
right hand key is the galvanometer key and is to 
be only tapped until a close balance is obtained. 
Otherwise were the bridge rheostat not adjusted 
to the measurement being made, the deflection 
would be violent. 

Much damage to a galvanometer is done by 
neglecting this rule. 

In a reflecting galvanometer where the mirror 
is suspended by a silk fibre, the latter may be 
broken. Replacing it is a work requiring expert 
knowledge and vast patience. 

Testing with the Bridge. For illustration of the 
use of the bridge the one shown in the last figure 
will be selected. 

Suppose it is desired to measure the resistance 
of a coil of wire. The two ends are bared of in- 
sulation, scraped clean and inserted in the binding 
posts as shown at x. 

The other connections of the figure are already 
made as described. For the first test, however, 
it will be well to use only one cell of battery. 

All plugs are in the holes in the rows of resist- 
ance coil connections. 



80 ELECTRICAL INSTRUMENTS. 

Remove from both A and B the plugs from the 
10 ohm coils. 

Remove a 10 ohm coil plug from the rheostat 
part. 

Depress the battery key and tap the galvano- 
meter key. 

The deflection is noted both as to amount and 
direction. Suppose it is 60° to the left. 

Remove another plug say from 100 ohm coil and 
tap key again. 

Deflection in same direction only 10°. 

Remove 20 ohm plug, deflection 5° to right. 
This indicates that too much resistance has been 
unplugged. 

Replace 20 ohm plug and remove 10 ohm plug, 
deflection is now 1° but to left. 

Indicates that still too little resistance is out. 

Remove 1 ohm plug; no deflection perceptible. 

Then read resistance unplugged. 10+ 100+ 10+1 
or 121 ohms is resistance of wire coil. 

When a resistance equal to that being measured 
is unplugged, no deflection takes place. 

It may be stated here that in these examples 
no actual statement of deflection due to resistance 
is meant. The values given are only illustrative. 

It will be seen that the deflection is to one side 
when too much resistance is unplugged and to 
the other side when too little resistance is unplugged. 

Most galvanometers fitted in the cases of port- 
able testing sets are marked + and - on opposite 
ends of the scale. 



THE WHEATSTONE BRIDGE. 81 

As the battery is connected so as to send current 
always in the proper direction, a deflection to the 
+ side means too much resistance and one to the 
— side too little resistance unplugged. Of course 
reversing the battery connections will reverse the 
value of these signs. 

In the bridge of the instruments being described 
in this section, there are three coils in each arm, 
reading from left to right 1000-100-10 in .4 and 
10-100-1000 in B. 

At present only equal ones are used in each arm. 
For measuring low resistances the 10 ohm coils in 
each arm are unplugged. For high resistances, 
100 ohm coils in each arm are used. 

For very high resistances 1000 ohms in each arm. 

The rule is to use the nearest bridge coils to the 
resistance being measured. 

As the total resistance of the rheostat is in this 
case only 11,110 ohms, it is evident that by the 
method before pursued, no higher resistance can 
be measured and none lower than 1 ohm as that 
is the lowest coil in the rheostat. 

Proportional Arms of Bridge. This is where the 
proportional arms of the bridge come in to use. 

By unplugging a higher coil in the B side, the 
value of each coil in the rheostat is multiplied by 
the number of times the unplugged coil in A divides 
into that of B. 

And b) reversing this and unplugging higher in 
A % the rheostat coils are divided by the number of 



82 ELECTRICAL INSTRUMENTS. 

times A is greater than B. As a memory aid — 
A divides, B multiplies. 

This gives new values to the rheostat coils and 
vastly extends the range of measurement. 

For example, let A be 10 and B 100, then B is 
ten times A ; multiply the rheostat figures by 10. 

The 1 ohm coil then becomes equivalent to a 
10 ohm coil and the 1000 ohm a 10,000 ohm coil, 
and so on. 

And let A be 10 and B 1000 and the rheo 
is multiplied by 100, the 1 ohm coil is thus equal 
to 100 ohms, the 1000 ohm to 100,000 ohms. 

This last setting of the arms will give a value 
to the rheostat of 11,110-100 or 1,111.000 
ohms. 

If then the bridge arms were so set and a balance 
obtained at 162 ohms, the actual resistance would 
be 162x100 or 16,200. 

In some bridges there are 1 ohm coils, then the 

ranee will be 1000 times and 777777-1— 
G 1000th. 

The dividing method or higher coil in A is on 
the same plan. 

Unplug 10 ohms in B and 100 in A, then the 
readings will be divided by 10, which is the number 
of times 10 goes into 100. And the 1 ohm coil 

becomes — ohm, the 10 ohm coil now being 

equal to a one ohm coil. 

This proportional property of the arms not only 
enables larger or smaller measurements to be made 



THE WHEATSTONE BRIDGE. 83 

than arc in the rheostat, but it gives closer read- 
ings. 

Take the first example, the coil of wire. Set 
the bridge arms A 1000, B 10, making a division 
by 100 necessary of the rheostat readings. As the 
first test showed 121 ohms, unplug one hundred 
times 121 ohms or 12,100 in the rheostat. 

As the bridge now stands this only equals 121 
ohms. It may be necessary to increase the bat- 
tery. 

Having depressed the battery key, depress the 
galvanometer key and there will probably be a 
deflection one way or the other. Suppose it is 
to the - side. 

Unplug 10 ohms now [ —J and try deflection. 

Let it take 18 ohms more to get a balance or zero 
on the scale. 

Then the rheostat will read 12,1 IS which di- 
vided by 100 gives 121. IS ohms, a far closer read- 
ing than with equal arms. 

It is a good plan to make a number of tests of 
coils of known resistance, or to check up tests by 
trying various combinations of tke arms. 

It must be remembered that as the resistance 
of copper increases from heat, readings will vary 
from time to time owing to the current flowing in 
the w T ire being tested. The relation between 
heat and resistance will be found elsewhere in 
these pages. 

In testing coils, electro magnets, etc., they must 



84 ELECTRICAL INSTRUMENTS. 

not lie near the galvanometer. Where leads are 
necessary from the bridge to the apparatus being 
tested, the leads also should be tested as their 
added resistance would give a false value to the 
test. 

Formula. A stating of the foregoing rules as 
formulas will be as follows: 

Let R be the resistance unplugged in rheostat, 
A that in arm A of bridge, B that in arm B of 
bridge, x the unknown resistance. 

Then A : B : :R : x t and A x = BR; therefore 
B R 



CHAPTER VI. 
Portable Testing Sets. 

The Willyoung Portable Testing Set— Model K. 

A form of portable Wheatstone Bridge is shown 
in Fig. 60. It is furnished with a small battery 
in the case, and a set of working shunts. 

The galvanometer is of the D'Arsonval type with 
a sensibility of one scale division for one volt 
through over two megohms. 

It is not disturbed by the proximity of other 
electrical machinery or magnetic fields. 

Full directions are furnished with each instru- 
ment. The general rules of its use are those for 
all forms of Wheatstone Bridges. 

In testing a resistance with this instrument it 
is connected to binding posts C D y Fig. 61. The 
flexible battery cords are connected by their cup 
connectors to two adjacent studs on the battery. 
This cuts in one cell; if more are desired the cups 
are connected to include them in circuit. 

The commutator plugs connect A x and B R as 
shown. Plugs G a and B a are inserted. Plug V 
is omitted, galvanometer switch is turned to " in " 

85 



86 



ELECTRICAL INSTRUMENTS. 



and shunt switch put on /, that is, if the particular 
set is equipped with a contained shunt. 

Plug all holes in the bridge arms but those corre- 
sponding to 100 ohms in each arm. 

Then unplug rheostat until enough resistance is 




Fig. GO. 



cut in circuit to equal what the resistance being 
measured amounts to. (When no idea is had of 
the latter, the galvanometer key must be tapped 
carefully that no undue deflection injures gal- 
vanometer) . 

Depress battery key and tap galvanometer key. 



PORTABLE TESTING SETS. 



87 



If needle swings to x, unplug more resistance. If 
to - cut out some of the coils in the rheostat. 




Fig. 61. 



By varying the proportions in the bridge arms, 
higher or lower resistances than those included in 



88 ELECTRICAL INSTRUMENTS. 

the rheostat can be measured. The latter runs 
from 1 ohm to a total of 21,110 ohms. 

The working range of this set is from .001 ohm 
to about 500,000 ohms with the battery supplied 
in the case. 

By using a larger battery connected at + and 
- at the right hand of the case, much larger read- 
ings may be made. And if a more sensitive gal- 
vanometer be connected where the shunt is, read- 
ings to a maximum of 21 megohms is claimed for 
this set. 

But for such high resistance work as the latter, 
the direct deflection method will be found ] >ivf ar- 
able. 

The Queen Portable Testing Set. The Queen 
Acme portable testing set, Pig. 62, is adapted for 

all resistance measurements. There are three rows 
of brass blocks and plugs controlling resistance 
coils. 

The middle row is the Bridge, the top and bot- 
tom rows, the rheostat. 

In the centre of the Bridge is a split block com- 
mutator R x which can be connected to the Bridge 
arms by plugs. 

If the plugs are inserted in this commutator in 
the direction of arrow L as shown, the resistance 
in the rheostat is divided by the quotient obtained 
in dividing the higher Bridge arm by the -lower. 

If plugs are in direction of arrow H, multiply 
rheostat by quotient. For example, let rheostat 



PORTABLE TESTING SETS. 



89 





Fig. 62. 



90 ELECTRICAL INSTRUMENTS. 

equal 100 ohms and commutator be set as in figure. 
100 ohms in arm B divided by 10 ohms in arm 

B = 10. Divide rheostat by 10, — - 10 

ohms. 

But let commutator be set in direction of arrow 

H. Then same example, -— = 10, 100 ohms 

in rheostat X 10 = 1000 ohms. 

The galvanometer scale is marked + and - . 
If the needle swings towards + , reduce the rheostat 
if to - , increase it. 

The range of the Bridge arms is 1 to 1000 ohms, 
and the rheostat 11,100 ohms. This gives a range 
of testing from .001 to 1 1 ,100, 000 ohms. For re- 
sistances above one megohm., however, more bat- 
tery is required than will be found in the case. 



Formula. For this Bridge, the formula for the 
commutator setting in direction of arrow L is 

— = -pr-, and for direction If/— = — 
BR B x. 

The general directions for this set are similar to 
the regular Wheatstone Bridge tests. It is very 
simple and easily handled. Other adoptions of 
the set will be found in later pages of this book. 

Table III. gives the Bridge setting for various 
resistance measurements. It is used in conjunc- 
tion with the directions regarding the commutator 
before given. 



PORTABLE TESTING SETS 91 

TABLE III. 

Showing setting of Bridge arms to measure re- 
sistances as in first column. 

" x" Ohms. Ohms Ohms. 

A B 

Below 1.5 1 1000 

From 1.5 to 11 1 100 

11 to 78 10 100 

78 to 1,100 100 1000 

1,100 to 6,100 100 100 

6,100 to 110,000 1000 100 

110,000 to 1,110,000 1000 10 

" 1,110,000 to 11,110,000 1000 1 



Whitney Testing Set. This portable set, Fig. 63, 
has 11,110 ohms in the rheostat and 1, 10, 100 
and 1000 ohm coils in each arm of the Bridge. 

One plug only is used in each row of the rheostat 
inserting the plug cuts in a resistance equal to the 
number marked on the block. 

The battery and galvanometer keys can be con- 
trolled by one button, the battery contacts being 
made first. Or they can be depressed separately, 
as desired. 

The inside connections are shown in Fig. 64. 

To Use Bridge. Connect the terminals of the 
unknown resistance to the lower right hand bind- 
ing posts. The terminal of the flexible cord pro- 
jecting from the hard rubber top between the 
bridge arms should then be adjusted to one of the 



92 



ELECTRICAL INSTRUMENTS. 




Fig. 63. 



r c 9^ 

— | L~p 












(5 6 (^ 5 



"""fj *y I 







/'"\JU^\±-f~~UJ^\JU — V-U — U-T^-U^-U^VU — ^ 




Fig. G4. 



PORTABLE TESTING SETS. 93 

five posts, likewise so located; thus throwing in 
circuit from one to five cells of battery, as will be 
seen by the diagramatic illustration herewith. 

If more battery power is needed remove the 
flexible cord terminal from the post and attach 
the terminals of an external source of direct cur- 
rent to the binding posts B B, the positive terminal 
being attached to the post marked " + ." 

This external e.m.f. should not exceed 15 volts. 

Assuming that nothing is known of the magni- 
tude of the resistance to be measured, insert the 
plug in the bridge arm lettered " multiplied by " 
in the gap numbered " 100 " and the plug in the 
bridge arm M divide by " in the gap numbered 
M 100." Place the plug in the " hundreds " row 
in the gap numbered " 1." The three remaining 
plugs numbered " " in their respective rows. 
This act has inserted 100 ohms in the rheostat 
arm of the bridge. 

Depress the combination key momentarily. 

If the galvanometer needle deflects towards 
" — " the rheostat resistance is too small and 
more should be added by moving the plugs along. 

If the deflection is towards " + " the rheostat 
resistance is too much and less is to be substituted 
by putting the " hundreds " plug into its " " 
gap and moving the " tens " plug along. 

Proceed in this manner until some combination 
of coil values is found where the galvanometer 
needle will no longer deflect when the key is de- 
pressed, 



94 ELECTRICAL INSTRUMENTS. 

The value of the unknown resistance is then 
indicated by the position of the rheostat plugs. 

The above applies only where the unknown re- 
sistance has a value between 1 ohm and 11,110 
ohms. 

If the resistance is lower than 1 ohm insert the 
plug in the left hand ratio arm into " 10 " and 
the right hand ratio arm plug into " 100 " or 
11 1000. M Balance may now be obtained as before, 
then 

n Multiply by v B 7) 

R - Tx . ' - ^ = \ or - - R = x. 
Divide by A 

For resistances higher than 11,110 ohms the plug 
in the left hand bridge arm must be inserted in a 
higher number than the plug in the right hand 
bridge arm. 

When using the set for the Varley loop as a 
Wheatstone bridge see that the plug in the " loop M 
bar is in block marked " V. & B." When using 
set for Murray loop test shift the plug into block 
marked " M." 

The lower right hand binding posts are used for 
all measurements with this set. 

The upper right hand binding posts are for 
extra battery. 

If an external galvanometer is to be used, it is 
to be connected to the left hand binding posts. 
The plug is shifted from the right hand hole in 
block marked galv. to the left hand hole in the same 



PORTABLE TESTING SETS. 



95 



block. This cuts out the galvanometer in the set 
and cuts in the external galvanometer. 

The Slide Wire Bridge. A simple form of Wheat- 
stone bridge is in Fig. 65. 

A piece of resistance wire about No. 24 B. & S. 
is stretched between two binding posts D E. Their 
distance apart is a little over one metre. 




rX-. 



-^v- 



[iiiii|iiiiii|iiiiii|iiiii|iiiii]|niiiiijiii!!ijiiiiii|iiiiii|iiiiii| !Jniiii|ihiiii}iimi|iiiimi}iiiiiiJ!i!iii|iiiii[jii. 





IN 



Fig. 65. 



Under this wire lies a scale graduated into 1000 
equal divisions with a zero at each end. 

Brass or copper strips of too small a resistance 
to be considered lie on the base connecting the 
binding posts as shown. 

The resistance to be measured is connected at x. 

An adjustable resistance is connected at A t 



96 ELECTRICAL INSTRUMENTS. 

partly taking the place of the rheostat in the P.O. 
and other Bridges. 

A battery and galvanometer being connected as 
shown, the slider C is moved along the wire D E 
until the galvanometer needle stands at zero. 

When the balance is obtained, the resistance of 
x will equal the result obtained by multiplying A 
by the length of wire between CE and dividing it 
by the length between D E. 

The length of the wire can be read either in 
millimetres or divisions of one thousand 

Formula. Let .v be the unknown resistance, R 
the resistance at A, A the length between D C and 

B the length between C E. Then R = — - — -. 

A 

The Stearns Bridge. An ingenious application 

of the Wheatstone bridge to the measurement of 

the resistance of bare wire in continuous lengths 
is found in the Stearns bridge. 

One each of the x terminals of a bridge is con- 
nected to a contact device consisting of a metal 
roller and a form of knife edge. 

The bare wire is wound from one drum on to a 
second drum passing in its passage through both 
of the contact devices. 

The bridge is adjusted to zero by making the 
rheostat equal the resistance of a length of wire 
between the contacts. 

The drums being then started, each successive 



PORTABLE TESTING SETS. 



97 



length passes the contacts and a continuous meas- 
urement takes place. 

The distance between these contacts being un- 
changeable, the resistance of the wire included 
between them should also remain unchanged. 

The Sage Ohmmeter. The Sage ohmmeter, Fig. 
66, is a portable slide wire Wheatstone Bridge of 




Fig. 66. 



peculiar pattern with a telephone receiver as well 
as a galvanometer. 

The adjustable rheostat takes the form of a fine 
wire, under which are marked resistances in vari- 
ous colors. 

A stylus 5 is connected to the telephone T as 
shown in Figs. 67 and 68 and can be touched at 



98 



ELECTRICAL INSTRUMENTS. 



any point on the wire rheostat. Plug P cuts in 
various resistance coils in the Bridge arm, the plug 
holes being colored to correspond with the markings 
on the scale. 




Fig. 07. 

In operation the resistance to be measured is 
connected to posts A and D. The telephone is 
held to the ear and the battery key on the tele- 
phone is closed. The stylus is tapped at various 
points on the rheostat wire until no sound is heard 




Fig. G8. 



in the telephone, the resistance being measured will 
be found in the figures at the point of stylus con- 
tact, where no sound was obtained. 

The numbers are to be read in that color corre- 



PORTABLE TESTING SETS. 



99 



sponding to the plug socket in use. If the plug 
is in a red hole, read the red figures, and so on. The 
plug must always be in one of the sockets when 
testing. 




The Evershed Testing Set. This simple testing 
outfit, Fig. 69, consists of an ohmmeter and a hand 



100 



ELECTRICAL INSTRUMENTS. 



generator. To test an insulation or other resist- 
ance, all that is required is to connect the resist- 
ance to the binding posts, turn the generator 
handle and read the resistance off the dial. No 
calculations whatever are necessary. 



^\ 




Fig. 70. 



The connections of the Evershed set are given 
in Fig. 70. 

Current from the generator D flows to the ohm- 
meter — h where it divides. Part travels through 
the pressure coil P and resistance R and tends to 



PORTABLE TESTING SETS. 101 

turn the magnetic needle towards the infinity mark 
on the scale. The other path of the current is 
through the current coil C and the fault x. 

If x has no appreciable resistance the current in 
C will be dependent only on the resistance of C 
and the e.m.f. of the generator. 

In this case the current coil will deflect the nee dl 3 
to zero, which is where the influence of each coil is 
equal. 

The resistance of x is in series with the current 
coil. It is obvious therefore that the more resist- 
ance in x the less current in C. And coil P turns 
the needle in proportion as its influence becomes 
strengthened by reason of the weakening of C. 

Increase of voltage from the generator does not 
affect the result as it affects both coils. 

The scale is graduated in ohms, the instrument 
being constructed with various sensibilities. 

The different instruments range from 2500 ohms 
to 5 megohms in one instrument and 25,000 ohms 
to 50 megohms in another. Sets for intermediate 
readings can also be had. 

The generators furnished range from 100 volts 
to 1000 volts in output. 



CHAPTER VII. 

Testing with Galvanometer. 

Current flow and e.m.f. in a Circuit. The cur- 
rent flow in all parts of a circuit is equal, but the 
e.m.f. varies according to where it is measured. 

In Fig. 71 a fine German silver wire is stretched 
between terminals .4 E and connected in series 
with a battery and an ammeter G\ F is a second 
ammeter connected in the circuit or G may be 
shifted. 

When the battery current flows through ,4 E, 
the ammeter placed at G or F shows the same 
reading, the current at G y F, or in fact in any part 
of the circuit is the same. 

Connect a voltmeter V at A and E and note 
reading, then connect V between A B, B.C, C D 
and D E and the latter four readings added to- 
gether will equal the one between A and E. 

If the wire A E is of uniform resistance, the 
e.m.f. between points at equal distances along it 
will also be equal. 

If V be connected as at first to .4 and the con- 
nection at E be drawn along the w T ire towards .4, 
the e.m.f. will be observed to decrease. This il- 

102 



TESTING WITH GALVANOMETER. 



103 



lustrates what is termed "the fall of potential," 
or if electrical work is being performed, the fall 
of e.m.f. 

If the voltmeter be connected across any two 
points and a greater current flow be permitted by 
means of an adjustable resistance R, the e.m.f. 
will rise with the current. 



HI- 



u-o 







G 



-^ 







Fig. 71. 



As long as the resistance of A E remains un- 
changed, a greater current flow requires a greater 
e.m.f. 



Applications of Ohms Law. If the resistance 
between the connections of V be known, the cur- 



104 ELECTRICAL INSTRUMENTS. 

rent flowing may be computed from the deflection 

E 

of V by Ohms law. / = — or the current equals 

K 

the e.m.f. divided by the resistance. 

For example, let A E measure 2 ohms and \ r 

4 

indicate 4 volts, then the current will be — 

or 2 amperes. 

If any two readings be known, the third can be 
computed by Ohms law. 

From the e.m.f. and current flow find the re- 

E 

sistance. R = -j- or the resistance equals the 

e.m.f. divided by the current. In this example 

4 

— = 2 ohms. 

And from the current and the resistance find the 
e.m.f. E = R xl or the e.m.f. equals the resistance 
times the current in this example 2x2 = 4 volts. 

The applications of Ohms law and the fall of 
potential will be found in the potentiometer, the 
shunt ammeter and in various tests to be described 
later. 



Testing Resistance. Let an unknown resistance 
be placed in series with a battery of constant e.m.f. 
and a galvanometer. Note the deflection and re- 
place the unknown resistance by an adjustable re- 
sistance. Adjust the latter until the second de- 
flection is equal to the first and the two resistances 
are equal. 



TESTING WITH GALVANOMETER. 



105 



If the adjustable resistance be of known value 
this method can be pursued, but a more practical 
method is to first ascertain the constant of the 
galvanometer. 

The " Direct Deflection " Method. This is the 
simplest method of testing resistances or insula- 
tions and is capable of extended application. 

It is based upon the fact that the greater the 
current flow through the galvano: later the wider 
the angle of deflection. 



-nNVWY 




/ \NVWV 

R 



Fig. 72. 



A known resistance R, Fig. 72, is put in circuit 
with the galvanometer G, battery B and double 
contact key K. 

After noting the deflection, the key is depressed 
and the unknown resistance x thrown in circuit. 
The second deflection is then noted and compared 
with the first. 

If the galvanometer deflections are proportional 
to the current, x will be as many times the resist- 



106 ELECTRICAL INSTRUMENTS. 

ance of R as the deflection through R is greater 
than that through x. 

For example, let R equal one hundred ohms 
and the deflection through it be ten degrees. The 
second deflection through x is twenty degrees. 
This shows that the current flowing through x is 
twice that which flowed through R and therefore 
the resistance of x is only one-half that of R, or 
fifty ohms. 

Formula. As a formula, let x equal the unknown 
resistance, R the known resistance, D the deflec- 
tion through R and d the deflection through x. 

Then x = -, — 

a. 

In case the galvanometer resistance is to be 

allowed for it is to be added to R but deducted 

from x. Calling it r, the formula stands 

D X (R + r) 

x = ^ r 

a 

Galvanometer Figure of Merit or Constant. It is 

customary in many uses of the galvanometer to 
determine the figure of merit or constant of the 
galvanometer. 

This is the resistance through which the gal- 
vanometer will give a deflection of one scale de- 
gree for one volt of e.m.f. 

If 100,000 ohms could be inserted in series with 
the galvanometer and one volt e.m.f. and still one 
degree of deflection be obtained, the figure of 



TESTING WITH GALVANOMETER. 107 

merit or constant of the galvanometer would be 
100,000 ohms. 

And if 50,000 ohms with one and one-half volts 
gave a deflection of three degrees, the constant 
would be as lollows: 

The e.m.f. is one-half as much again as required, 
therefore the deflection will be one-half as much 
again or one-third more than it should be. Re- 
ducing it by one-third gives two degrees of deflec- 
tion. 

And it is evident that if two degrees are obtained 
through a certain resistance, one degree would be 
obtained through twice the resistance. 

One degree of deflection only requires one-half 
the current to produce it that two degrees does, 
therefore the constant for the galvanometer is 
50,000x2 or 100,000 ohms. 

Formula. To make a formula out of this let D 

be the deflection through the resistance, R be the 

resistance,, V the e.m.f., and x the constant re- 

. D XR 3x 50,000 innnnn 
quired. Then — =7 — = x or n = 100,000. 

The terms constant, figure of merit and sensi- 
bility are used to- mean the same. Sensibility or 
constant are- the most generally adopted, however. 

With Shunts. If a shunt is used in obtaining 
the constant the deflection and resistance must 
be multiplied by the value of the shunt which 
will be known as n ? as before. 



108 ELECTRICAL INSTRUMENTS. 

In the above example let the shunt be — 

with a multiplying power of 10. 

The formula will be then 

DxRxn JA , , , 3x50000x10 
— = x and the last example — 

V 1 7£ 

or 1,000,000 ohms. 

Using the — shunt only one-tenth of the 

current went through the galvanometer but as 
it is for example assumed to give the same de- 
flection as at first, its constant is ten times higher. 

In some cases the full battery perhaps 100 cells 
is used to get a constant by shunting the galvano- 
meter. 

The constant is then not for one cell but for the 
whole battery. 

Its value will be the product of the deflection, 
the known resistance and the shunt. 

Formula. Using the above letters, (battery of 
one hundred cells) , 

DxRxn = constant. 

Example. Let the deflection be 16°, resistance 

100,000' and shunt — -r~. Then constant for the 

battery in use will be 16x100,000x100 or 160 
megohms. 

And it is evident that the constant for one cell 
will be one-hundredth of the above, or 1,600 000 
qjims. 



TESTING WITH GALVANOIMETER. 109 

Where the figures of resistance become high it 
is preferable in calculating to substitute the value 
in megohms or fractions of a megohm. 

Second Example. Let the figures used in de- 
termining the constant be, R = .1 (one-tenth) of 

a megohm, D be 100° and the shunt . 

In testing an unknown resistance with the same 
shunt and battery the deflection is 10°. 

Then the constant is 100 X.lx 1000 or 10,000 
megohms. 

a 1 .i 1 10,000 innn 

And the unknown resistance — — — or 1000 

megohms. 

Summary of Rule. When using the same shunt 
battery and galvanometer the value of an unknown 
resistance will be determined by dividing the con- 
stant by the deflection obtained through this un- 
known resistance. If a different shunt J is used 
the formula will be 

DxRxn 
dxs 

Deflection Constant. It is often convenient to 
use the deflection obtained through a known re- 
sistance as the constant. The degrees of deflection 
obtained in a test will then be calculated in terms 
of the resistance used when getting the constant. 

For example, let the deflection through one 
megohm be 85 degrees. Then the resistance-con- 



110 ELECTRICAL INSTRUMENTS. 

stant would be one degree through 85 megohms, 

and the deflection-constant would be 85 degrees 

through one megohm. 

In testing an unknown resistance under the 

same conditions of battery, shunt, etc., the de- 

85 1 

flection is 170 degrees. Then — — = — and the 

unknown resistance is one-hall megohm. 

Rule: Divide the deflection-constant by the de- 
flection through the unknown resistance; answer 
is in terms of standard used to determine constant. 

Formula. Let R be the standard resistance, D 
the deflection through it, x be the unknown re- 
sistance, d the deflection through x. Then 

DxR 

If R be one megohm the calculation is simplified 
for quick working, and becomes -j in megohms. 

Direct Deflection with Queen Set. The Queen 
Acme set, T 460, may be used for tests of this 
nature; a special set is, however, constructed hav- 
ing a resistance in series with the galvanometer. 
The latter can also be shunted by manipulating 
the Bridge. 

The T 460 set is used as follows: 

To obtain the constant, connect a resistance of 
about 100,000 ohms such as one of the glass slabs, 



TESTING WITH GALVANOMETER. 



Ill 



between the top left hand post and the + battery 
post. 

Remove all plugs from the commutator between 
the Bridge arms and plug in all coils. Connect in 
one cell of battery by means of the flexible cords. 

The setting should be as in Fig. 73. It will be 
seen from this diagram that the circuit starting 
downwards from the slab runs through the + bat- 
tery post, battery cell, battery key B a, rheostat 




Fig. 73. 



blocks, galvanometer, galvanometer key G a, to 
the main post and to the upper part of resistance 
slab. 

The slab, galvanometer and battery will thus be 
in series when B a and G a are depressed. 

If any plugs have been left out in the rheostat, the 
resistance of the coils they control w T ill also be in- 
cluded in the circuit. 



112 ELECTRICAL INSTRUMENTS. 

When B a and G a are depressed a deflection of 
the galvanometer will ensue. If this deflection is 
one degree, the constant is 100,000 ohms. 

If more than 1 degree, remove plugs from rheo- 
stat and add resistance thereby interpolated tc 
resistance of slab. The constant will be the figure 
so obtained. 

And if less than one degree, a lower resistance 
than 100,000 ohms must be used, but it is not 
likely that this will be the case. 

The constant having been obtained, detach slab 
and connect resistance or insulation to be meas- 
ured in its place. 

If the deflection is too small add more cells. A 
larger battery can be added by connecting it to 
the battery posts, detaching the flexible cords and 
cups from the battery in the case. 

To determine the resistance now being measured, 
divide the constant by the deflections obtained and 
multiply the result by the number of cells used. 

For example, let the deflection be 10° with 5 

n 100,000 X 5 nnn 

cells. —r = 50,000 ohms. 

The deflection has been increased 10 times, 
which would show that the resistance was only one- 
tenth of the constant or 10,000 ohms. 

But as five times the e.m.f. has been used, the 
result must be multiplied by five. 

Another method is to use the deflection obtained 
through 100,000 ohms as the constant. Divide this 
constant by the deflection obtained in the te^t and 



TESTING WITH GALVANOMETER. 113 

multiply by the number of cells used. The answer 

will be in terms of 100,000 ohms. 

For example, let 8° be the deflection constant 

through 100,000 ohms with one cell, and 4° be that 

through the unknown resistance with five cells. 

8 
Then — x 5 X 100,000 = the unknown resist- 
4 

ance, or 1,000,000 ohms. 

In such tests the resistance of the galvanometer 
may be neglected. 

If it is desired to use only the galvanometer of 
the Acme set, remove the plugs from the com- 
mutator, insert all other plugs and connect to the 
main posts at left end of case. The galvanometer 
and key are now directly connected to these bind- 
ing posts. 

Measuring Resistance with Voltmeter. The volt- 
meter being considered merely as a galvanometer, 
it may be substituted for the latter. 

The deflection obtained from a battery or other 
source of current is noted. 

This deflection is actually through the resistance 
of the voltmeter. 

A second deflection is then taken with the un- 
known resistance in series with the voltmeter and 
battery. 

This deflection is dependent upon the current 
allowed to flow through both the voltmeter resist- 
ance and the unknown resistance. 

The first deflection multiplied by the voltmeter 



114 



ELECTRICAL INSTRUMENTS. 



resistance will give the resistance through which 
one degree of deflection could be obtained, or the 
constant. 

The value of the second deflection can then be 
calculated from this constant, the latter divided 
by the second deflection will give the total resist- 
ance. Subtract the voltmeter resistance and the 
result will be the value of the unknown resistance. 

The connections are as in Fig. 74, more extended 




Fig. 74. 

description of this test being given in the chapter 
treating on measurements with the voltmeter. 



Testing Resistance of Galvanometer. In many 
tests it is necessary to know the resistance of the 
galvanometer being used. If it is not marked on 
the instrument it may be found by the following 
method : 

Arrange the galvanometer G, Fig. 75, in series 
with an adjustable resistance R and battery C. 



TESTING WITH GALVANOMETER. 115 

The resistance and battery should be so selected 
that the deflection of G is about one-half the scale. 
This is to enable readings to be made with accuracy. 

Having noted first deflection, increase resistance 
R until deflection of G is just one-half of its first 
deflection. 

The battery used must be constant or its varia- 
tions will affect result. They may, however, be 
compensated for. Battery resistance is omitted. 

The resistance of the galvanometer will equal 
the resistance of R as measured at the time of one- 
half deflection less twice the original resistance of R. 




i 
i 

•» — -+■ — J 

Fig. 75. 

For example, let R equal 10,000 ohms and the 
first deflection be 40°, increase R until at one-half 
deflection it equals say 25,000 ohms. Then 10,000 
X2 or 20,000 from 25,000 leaves 5000 ohms, the 
resistance of the galvanometer coils. 

Formula. As a formula, let R be first resistance, 
r be second resistance ; then G = r - 2 R. 

Resistance of Galvanometer by Bridge. Connect 
the galvanometer G between the posts used in or- 
dinary resistance tests, as in Fig. 76. 



Ll6 ELECTRICAL INSTRUMENTS. 

Adjust the rheostat R until the needle of G is at 
the same deflection when key is open or closed. 
Then the resistance of G will equal resistance in R 
multiplied by result obtained from dividing re- 
sistance in arm B by that in arm A. 

Battery Tests. Tests of battery cells may be to 
determine e.m.f., internal resistance or life. For 
the latter no directions will be given. 



The simplest test for e.m.f. is to connect a volt- 
meter across the terminals of the cell. A high re- 
sistance voltmeter must be used, one with not less 
than 80 ohms per volt is desirable. 

The cell being disconnected from any other cir- 
cuit will give its e.m.f. on open circuit. 

Tests of similar nature may be made by dis- 
charging cell through resistances of different values. 

Tests for internal resistance are needful in many 
electrical operations and a number of methods will 
be described. 



TESTING WITH GALVANOMETER. 



117 



First Method of Measuring Resistance of a Cell. 

Take open circuit e.m.f. of cell with voltmeter of 
known resistance R. 

Add resistance r in series with cell and volt- 
meter and adjust until e.m.f. deflection is one-half. 

Calculation: Add additional resistance to that of 
voltmeter so as to obtain total resistance in series 
with cell. Subtract twice the resistance of volt- 
meter and answer will be that of cell. This test 
is only suitable where internal resistance of cell 
is high. 



r~iiH 


i 
i 
i 


1 1 


i 


I c 


1 ff K • 


+ 

1 
1 

1 
1 

___ — —_____ — i — _ 




Fig. 77. 

Example. Voltmeter = 521 ohms, e.m.f. 1.5 
volts; adding 530 ohms reduces e.m.f. to .75 volt. 
Total resistance 521 + 530 = 1051. This less 521 x 
2 or 1042 = 9 ohms. 

Formula. (R + r) - 2 R = cell resistance. 

Second Method. In Fig. 77 C is the cell whose 
internal resistance it is desired to measure, V a 



118 ELECTRICAL INSTRUMENTS. 

voltmeter, R a known resistance and K a key to 
connect R across the circuit when desired. 

First read the e.m.f. of C with the key open; 
then depress key and read the e.m.f. across R. 

Then the resistance of the cell will equal the 
first e.m.f. minus the second e.m.f. divided by the 

second e.m.f., or as a formula, r = R x -. 

The second e.m.f. is really the drop across the 
resistance. 

Example. — Let first e.m.f. be 2 volts, resistance 
be 2 ohms, second e.m.f. across resistance be 1.5 
volts, then internal resistance is 

2-1.5 .5 ,1 2 . 
2 X — 7-z — or 2x— or2 x— = — ohm. 
1.5 1.5 6 S 

Third Method. Another shunt method is by 
using a resistance which equals the total resist- 
ances in the circuit such as galvanometer, wires, 
etc. Connect this in shunt across the cell, also 
connect galvanometer and wires to which shunt 
was adjusted across cell. 

The connection would be similar to the last 
figure, only the shunt should be directly connected 
across C. 

Having made these connections, note deflection, 
and then remove shunt by opening key. 

Note second deflection. Add resistance in gal- 
vanometer (or voltmeter) circuit until second de- 
flection equals first. 



TESTING WITH GALVANOMETER. 119 

The value of the added resistance will be the 
internal resistance of the cell. 

At the first deflection, there was a two branch 
divided circuit of equal resistances, the current 
then through galvanometer was one-half the total 
flowing.. 

At the second reading the added resistance cut 
down the current by one-half, but by a series re- 
sistance instead of a shunt. 

Fourth Method. The simplest test of internal 
resistance is with the voltmeter and ammeter. 
Connect ammeter so as to obtain reading of cur- 
rent using a resistance to control current. 

Take e.m.f . reading directly across cell terminals. 

E 
Then by ohms law R = — 

In making this last test the current from the 
cell may be reduced which is an advantage by 
adding a resistance in series with it. The am- 
meter is to be included in the circuit. 

The e.m.f. V of the cell on open circuit is first 
read. With the resistance in, the e.m.f. is again 
read v v The current (/) is also read on the 
ammeter. 

Then r or the resistance of the battery is equal 

to— \ 

Most of the above methods are only suitable for 
non-polarizing cells or cells of low internal resist- 



120 



ELECTRICAL INSTRUMENTS. 



ance. The Wheatstone Bridge methods are pre- 
ferable. 

Resistance of Battery by Bridge. In Fig. 78 C 
is the cell to be measured connected to line posts 
of bridge. 

A key K and the galvanometer G is connected 
as shown. 

Make the reading of G equal whether K is open 
or closed by adjusting rheostat R. 




Fig. 78. 



Formula. Then the resistance of C will equal 

T> 

the result obtained by multiplying R by — 

In the formula R is resistance of rheostat, B 
that of arm B of bridge, and ^4 that of arm ^4 of 
bridge. 

Measuring Current Flow of Cell. The rate of 
current flow of a cell may be deduced from Ohms 



TESTING WITH GALVANOMETER. 121 

law after reading the e.m.f. and the internal re- 
sistance. 

But it may also be measured with an ammeter 
on short circuit. 

The latter method is not always desirable. The 
test can also be made with a tangent galvanometer. 

The tangent galvanometer can be used as an 
ammeter providing the directive force of the 
earth's magnetism be known for the place where 
the measurement is made. 

This directive force is used in the form of a con- 
stant. It varies however, from time to time. 

The constant of the place of measurement is 
multiplied by the radius of the galvanometer coil 
in inches and the result divided by the number of 
turns in the galvanometer coil. This gives a con- 
stant for the galvanometer. 

When a current produces a deflection of the 
needle, the tangent of the angle of deflection mul- 
tiplied by this galvanometer constant equals the 
current flow in amperes. 

Example. — A cell of battery applied to the ter- 
minals of a 10 turn coil which had a radius of 6 
inches produced a deflection of 1S°. The test was 
made in New York. 

Constant of the galvanometer equals - — — — 

or .4464. 

Tan (tangent) 18° = .3249. 

Then .4464 x. 3249 = 1.4518 amperes. 



122 ELECTRICAL INSTRUMENTS. 

Formula. Let I = current flow; H constant of 

location ; r radius of coil ; N number of turns of coil ; 

G constant of galvanometer; T tangent of deflec- 

H X r 
tion. Then G = — r^— and / = G X T, or 

iV 

r Hx r 



CHAPTER VIII. 
The Potentiometer. 

The Potentiometer. Referring back to Fig. 71 
let the negative terminals of two independent 
battery cells be attached to A and their positive 
terminals to E. No current will flow along .4 E 
if the e.m.fs. of the cells are equal. This can be 
proven by inserting a voltmeter in series with 
either cell or between A E. One e.m.f . is opposing 
the other e.m.f. If now two cells in series be ap- 
plied to A E and a third cell be connected in mul- 
tiple between .4 E as before, a voltmeter or gal- 
vanometer will show current due to the excess of 
e.m.f. from two cells against one. Or if all the 
cells are equal, the deflecting force will be equr 1 
to that of one cell. 

Connect a voltmeter or galvanometer in series 
with the one cell and leaving the negative ter- 
minal connected to A move a w r ire from the volt- 
meter along W. A point will be reached where 
the voltmeter or galvanometer will settle to zero 
showing the e.m.f. of the one cell is equal to that 
opposing it from the tw r o cells. As the wire W is 
of uniform resistance, the e.m.f. across one-half of 

123 



124 



ELECTRICAL INSTRUMENTS. 



it will be one-half that across its entire length. 
The point of zero will then in the present case be 
in the middle at C, and the relation between the 
two lengths of wire will equal that between the two 
e.m.fs. Or the length A C will be to .4 E as the 
e.m.f . of the one cell will be to that of the two cells. 
Although there is some similarity between a 
potentiometer and a Wheatstone bridge there exist 
two great differences. The bridge has one source 




Fig. 79. 



of e.m.f. and two circuits; the potentiometer two 
opposing e.m.fs. in one circuit. 

A balance is obtained in the bridge when the 
points of equal e.m.f. in two circuits are connected 
through the galvanometer. 

In the potentiometer the point of equal e.m.f. 
between two e.m.fs. is found in one circuit. 

A simple form of potentiometer is shown in 
Fig. 79, Between the binding posts A B is 



THE POTENTIOMETER. 125 

stretched a fine resistance wire. A sliding con- 
tact P moving on a guide rod carries an index 
which indicates on a scale graduated into 1000 
equal divisions. This scale may conveniently be 
one metre long and the wire No. 24 B. & S. gauge. 
A standard cell 5 is connected, one terminal to A 
and the other terminal to the voltmeter V. The 
second terminal of the voltmeter goes to the slid- 
ing contact P. If V is not provided w T ith a kev 
one may be inserted between A and P. The cell 
the e.m.f. of which is to be compared is connected 
to A and B. Similar terminals of each cell must 
be connected to A. 

Contact P being moved, 5 and C being both in 
circuit, a point on the wire will be found where V 
gives a zero reading. 

When the balance is obtained, the scale divisions 
from A to P will bear the same relation to those 
from A to B that the e.m.f. of cell 5 does to the 
e.m.f. of C. The e.m.f. of 5 being known the 
e.m.f. of C is easily calculated. 

Let 5 be 1.019 volts ,\4 P be 755 and A B 1000. 

mi *rr i^r, , r*i ^ • ^ 1000x1.019 

Then 755:1000 : : 1.019 is to C or -— = 

i oo 

1.349 .... volts. 

HAP were 10 and A B 1000, C would equal 
101.9 volts. It is evident that 5 should be less 
than C or no point of zero will be found. 

As this is a zero method, that is the object being 
to reduce V to zero, V need not be calibrated in 
volts but may be any form of galvanometer. If 



126 ELECTRICAL INSTRUMENTS. 

it were desired to check a voltmeter, the latter 
would be inserted across A B, the reading on this 
voltmeter should equal the e.m.f. obtained by the 
calculation. 

As the sliding of the contact P only cuts in or 
out a resistance formed by the wire, the re- 
sistance of the latter may be determined. Each 
scale division will then have a value in ohms or 
fractions of an ohm. 

To make this clear, let the wire be for example 10 
ohms, then each of the 1000 divisions on the scale 

will equal — — of an ohm. 

And in the foregoing example 755 divisions 
would equal 7.55 ohms. 

The whole example would read 7.55 ohms : 10 
ohms : : 1.019 is to 1.349 volts, the same result as 
before. 

To check an ammeter it would be placed in 
series with a steady source of current. A shunt 
or known resistance is also in scries with the am- 
meter. 

Leads of no appreciable resistance are run from 
each end of the shunt to A and S, The e.m.f. 
across this shunt is first determined. The current 
flowing is then calculated by dividing the e.m.f. 
across the shunt by the resistance of the shunt, or 

/ = — This should equal the reading on the 
K 

ammeter. The shunt must be of sufficiently high 

resistance to give a drop of e.m.f. greater than 



THE POTENTIOMETER. 127 

the e.m.f. of 5. For example, let the shunt be 
5 ohms, and the e.m.f. across it be 10 volts. Then 

— = 2 amperes. Some of the foregoing rules 
o 

may be modified in actual work with the more 
complicated forms of potentiometers, but the prin- 
ciples remain the same. 

If it is desired to measure a high e.m.f. and yet 
not apply it in full to the potentiometer, any 
desired e.m.f. may be taken off by means of re- 
sistances. A number of resistance coils being 
placed in series with the high e,m.f., connection 
may be made between two or more for the desired 
e.m.f. In the case of an e.m.f. of 100 volts, let 
10 volts be needed for purposes of measurement. 
Place in series two coils one of 100 ohms and one 
of 900 ohms. Connecting across the 100 ohm coil 
will give an e.m.f. of 10 volts, across the 900 ohm 
coil, 90 volts, and across the two coils, 100 volts. 
The reason for using such high resistances is to 
keep down the current. The actual resistances 
used may vary according to circumstances, their 
relation to one another is only of importance. 

In some tests it is desirable to connect the 
known source of e.m.f. across .4 B and the un- 
known across A P. 

By using a charged storage battery and suitable 
high resistance a steady e.m.f. may be obtained 
for some hours. Dry cells are not suitable as 
their curve of e.m.f. drops too abruptly. 

In using the potentiometer with a large battery 



128 ELECTRICAL INSTRUMENTS. 

of constant e.m.f. the latter is connected to A B, 
its positive pole to A. 

A standard cell 5 is connected, positive pole to 
A and negative pole through voltmeter or galvano- 
meter to P. 

P is moved until there is no deflection. The 
divisions between A and P are noted, or if the 
scale is graduated in terms of resistance of W, the 
ohms are noted. Call this R. 

The standard cell being replaced by the cell 
under test, a point of balance is again sought. 
Call the resistance of divisons from .4 to P in this 
case r. Then the e.m.f. of the cell being tested 
is to the e.m.f. of 5 as r is to R. 

It may be necessary in order to obtain greater 
accuracy to use two or more standard cells in 
series between A and P. 

For example, let e.m.f. of the standard cells 5 be 
2.038 A B equal 1000 and A P equal 100. Then 

the e.m.f. across A B will be — — X 2.038 or 20.38. 

Substituting the cell C to be tested for 5 it is 
evident that zero balance will be again obtained 
when its e.m.f. balances the e.m.f. between .4 P. 
It may be then read in terms of the e.m.f. across 
A B or in terms of S. 

Suppose balance now is obtained at 125. This 

will be -^ of A'B or 2.547 likewise ~ of 5. 
In figures ^ X 20.38 = ^X 2.038. 



THE POTENTIOMETER. 129 

The modern testing sets are generally suitable 
for use as potentiometers as the following examples 
will show. 

As plug switches are used instead of sliding con- 
tacts, the relations corresponding to divisions on 
a scale will be those of resistances in ohms. In 
the testing sets, the resistance equivalent to that 
between A P, Fig. 79, is made 100 ohms and is 
not changed. The resistance equal to P B is 
adjustable and is first made high. 

As the sliding of P merely changes the relation 
between A P and A B the same result is obtained 
by changing only P B. This which is not feasible 
in the slide wire instrument becomes an advantage 
in the plug switch pattern. 

In the following two tests consider the B arm 
to represent the wire A P and the rheostat the 
wire P B. The B arm plus the rheostat will then 
equal the total resistance of wire W or from A to B. 

Although the examples given are measurements 
of e.m.f. or current, the potentiometer is equally 
well adapted for comparing resistances and insula- 
tions. 

Checking Voltmeters. Voltmeters may be 
checked by comparison with a standard instrument 
or by the potentiometer method. 

In the former case, the standard instrument 
and the one being checked are connected in mul- 
tiple across the e.m.f. at the same point and their 
readings compared. 



130 ELECTRICAL INSTRUMENTS. 

Potentiometer Method with Queen Set. Unplug 
10,000 ohms from rheostat, unplug A arm of 
Bridge and unplug 100 ohms only from B arm. 
Remove plugs from commutator except one in 
upper right hand hole, that is, connecting R and 
B blocks. 

Disconnect flexible battery cords from battery 
tips. Connect negative terminal of a standard 
cell or cell of known e.m.f. to line post C and its 
positive terminal to + battery post. 

Connect line e.m.f. to battery posts, + to + 
and - to - . 

Depress B a and G a keys and change resistance 
in rheostat until no deflection occurs. If needle 
goes towards + on scale, reduce rheostat and vice 
versa. Proceed carefully as the balance will occur 
at a point of slight change in the rheostat. 

When balance is obtained, add 100 ohms to the 
reading of the rheostat and divide by 100 or point 
off two places. 

Multiply this result by e.m.f. of standard cell 
and answer is e.m.f. of outside circuit. 

For example, suppose balance was obtained with 
8855 ohms in rheostat and e.m.f. of standard cell 
is 1.44. 

Then -r— ■ = 89.55 and this multiplied by 

1.44 = 128.95 volts as e.m.f. of outside circuit. 

Formula. Let R = rheostat at balance; B = 



THE POTENTIOMETER. 



131 



resistance of B arm, e = e.m.f. of standard cell, 
and E, e.m.f. to be found. 

Then — - — e = £, or what is the same thing, 



B 



R + B 
B 



Xe 



E. 



Potentiometer Method with Willyoung Set. The 

connections for the measurement of an e.m.f. by 
the potentiometer method are shown in Fig. 80. 




R A 



Sourc* o^HltXT, 



Fig. SO. 



The operation to be performed here is the check- 
ing of a voltmeter. 

5 C is a standard cell, .4 one arm of the bridge, 
R the rheostat. 

A key and shunt are in the galvanometer circuit 
and a key or switch in the power circuit. The 
latter may be a tap from a direct current main or 



132 



ELECTRICAL INSTRUMENTS. 




Layout of Fig. 80. 



THE POTENTIOMETER. 133 

storage battery. But a few lamps or other resist- 
ance should be put in circuit to avoid danger of 
excessive current. 

The standard cell negative terminal is connected 
to C and the positive terminal to a movable plug 
which for the present is placed in the 100 ohm hole 
of the A arm. 

The outside e.m.f. is connected, positive ter- 
minal to + battery post and negative terminal to 
- battery post. R is then adjusted until needle 
remains at zero, always closing the battery key 
first. 

R should be first made very large and gradually 
reduced. Unplug say 20,000 ohms and then re- 
place plugs one by one. 

On a circuit of 110 volts R will be somewhere 
near 9000 ohms. 

It will be found that at a certain point a very 
slight change in the rheostat will reverse the gal- 
vanometer deflection. 

To determine the e.m.f. of the line, which should 
be also the reading on the voltmeter if it is accurate, 
figure as follows: 

Add 100 (the resistance in the B arm) to the 
resistance needed in the rheostat for a balance and 
point off tw^o places, that is, divide the above sum 
by 100. Multiply the result by the e.m.f. of the 
standard cell and the result is the e.m.f. of the line. 

For example, suppose 7S16 is the rheostat read- 

7816 + 100 



ing, 1.44 the e.m.f. of cell. Then 



100 



134 ELECTRICAL INSTRUMENTS. 

79.16 and X 1.44 = 113.99 volts. The difference 
between this result and indication on voltmeter is 
the error of the latter. 

Of course if the standard cell is one volt, the 
e.m.f. would be 79.16 or the rheos ; at reading plus 
the B arm and divided by one hundred. 

Formula. Let E represent e.m.f. of line (or 
indication voltmeter should show), c the e.m.f. of 
standard cell, R resistance in rheostat, B resistance 
in B arm. 

Then E = — ==— x e. 



CHAPTER IX. 
Condensers. 

Charge and Discharge of Condenser. Connect a 
condenser C, battery Z?, galvanometer G and key 
K as in Fig. 81. 

Depress K and a deflection will be noted at G. 
This is due to the charge of the condenser. 

Release K and upon the back contacts meeting, 




Fig. 81. 

a second deflection takes place at G but in the 
opposite direction. 

Again depress K so as to open the back contact 
but not close the battery circuit. Upon again re- 
leasing K a deflection will ensue but very much less 
than the previous ones. And this last operation 
being repeated, decreasing deflections will be noted. 

*35 



136 



ELECTRICAL INSTRUMENTS. 



If K be depressed for varying intervals of time 
and released the deflections will also vary. 

It takes a certain length of time to charge a 
condenser. Increasing the battery will reduce the 
time of charge. 

If a Kempe discharge key is used the connections 
will be practically the same. The back contact 
of the key in the figure will be the top contact in 




Fig. 82. 



the Kempe key, and the front contact the lower con- 
tact of the Kempe key. 

The capacity of the condenser is measured by 
the extent of the deflection upon first short cir- 
cuiting it through the galvanometer. This is true 
when battery power and other conditions are 
equal. 

If the discharge of C be observed with an ap- 
propriate apparatus, it will be seen to be oscillatory 
as in Fig. 82, not steady as that of a battery cell. 



CONDENSERS. 137 

A condenser does not store up electricity, the 
operation called charging merely leaves it under 
an electric strain which is relieved when it is dis- 
charged. 

It may be likened to a spiral spring which is 
suspended from one end and has a weight at the 
other end. If this spring be pulled still farther 
down, an ability to do work or a potential is given 
it. Now upon releasing the spring, it contracts, 
pulls up the weight, and then slightly expands 
again, causing the weight to jump up and down 
until the force is exhausted. 

The spring and weight have an oscillating mo- 
tion even as does the condenser discharge show an 
oscillatory character. 

A condenser offers the resistance of its insula- 
tion to a direct current, in other words will not 
allow a current of this character to pass through it. 

But the effect of an alternating current is not 
arrested by a condenser. An ordinary vibrating 
bell will not be rung by a battery through a con- 
denser except one tap due to charge. The alter- 
nating current of a telephone magneto will ring 
the bell with which it is equipped without trouble 
through a condenser. 

This does not show, however, that the alternating 
current actually passed through the dielectric. It 
is the inductive effect between the plates, a con- 
denser acts therefore by the induction taking place 
between its parts. 

To avoid complications of words the condenser 



138 ELECTRICAL INSTRUMENTS. 

is said to retain a charge and to permit itself to 
be discharged. 

Measurement of Capacity. The capacity of a 
condenser is measured by comparing the discharge 
deflections with those obtained from the discharge 
of a condenser of known capacity. The latter is 
termed a standard condenser. 

The operation is as follows: 

Charge standard (5) for a certain length of 
time, say one minute, then discharge it and note 
deflection (D). 

Then replace standard by the condenser under 
test (C). Charge and discharge under precisely 
similar conditions and note deflection (d). 

The connections may be similar to Fig. 81 or the 
battery may be connected directly to the con- 
denser through the key. The galvanometer would 
not be then in circuit during charge. Or the gal- 
vanometer may be shunted or short circuited. 

The condenser under test will bear the same re- 
lation in capacity to the standard that the deflec- 
tions do to those obtained by discharging the latter. 

Formula. Or as a formula, 

5 :C ::D :d. ThenC = S^. 

Another method by the bridge is as follows: A 
standard condenser and the condenser to be 
tested are connected in a similar manner to the 



CONDENSERS. 139 

two arms of the bridge. Each condenser takes 
the place of the coils in one arm. 

An adjustable resistance is connected in the 
binding posts provided for the x or unknown re- 
sistance. 

A discharge key and battery are connected 
taking the place of the regular battery in the 
bridge. The condensers are charged and dis- 
charged, and the resistances adjusted in the other 
sides of the bridge until no deflection obtains. 

At balance the two condensers are to each other 
in respect to capacities inversely as the two re- 
sistances are to each other. 

Example. Capacity of standard condenser 5 is 

.3 (three-tenths) microfarad. Resistance R on 

same side of the bridge is 1800 ohms. Resistance 

r on side of bridge where condenser T to be tested 

is connected, is 1200 ohms. Then 7:. 3 : : 1800- 

1onn 3x 1800 4K .' - A 

1200 or — —--r — = .45 microfarad. 

Formula. Using above letters, T: S : : R: r. 

Insulation of Condenser. A condenser can be 
tested for insulation by charging it through a 
shunted galvanometer. When fully charged the 
deflection should return to zero. 

If a deflection is shown after charging is com- 
pleted it indicates that current is passing through 
condenser. The value of this current and the re- 
sistance of the fault in the condenser are computed 



140 ELECTRICAL INSTRUMENTS. 

from the galvanometer constant as in any direct 
deflection method. 

But this method does not give as close a test of 
very high insulations as the loss of charge method. 

Insulation by Loss of Charge. A condenser re- 
tains its charge (or electric strain) for a length of 
time dependent upon the perfection of its insula- 
tion. This property gives a method for deter- 
mining its insulation as well as by the usual resist- 
ance tests. 

A condenser is attached to a battery for a given 
length of time and then discharged through a gal- 
vanometer. The deflection having been noted, the 
condenser is again charged under precisely similar 
conditions and left with its terminals insulated 
for a given period of time and again discharged 
through the galvanometer. 

The two deflections being compared the loss of 
deflection in the second reading is a measure of the 
loss of charge. This loss being due to the state 
of insulation, the latter is therefore determined. 

In a good standard condenser this loss for a 
short period of time is inconsiderable. 

As cables and wires act in a similar manner to 
condensers, their insulation can also often be 
determined by the loss of charge method. 

The following figures are approximate as the 
actual substance or fluid* varies somewhat. For 
purposes of comparison, however, the figures are 
sufficiently accurate. 



CONDENSERS. 141 

TABLE IV. 
Specific inductive capacity taking air as 1. 

Flint Glass 10 

Plate Glass 7.06 

Gutta-percha 2.62 

Mica 5 

Paraffin wax 2.31 

Pitch 1.80 

Shellac 2.59 

Sulphur 3.82 

Olive oil 3.16 

Castor oil 4.78 

Turpentine 2.23 



CHAPTER X. 

Cable Testing. 

Cable Testing. If a resistance be shunted across 
a condenser it will be found that no steady deflec- 
tion can be obtained until the condenser is charged. 

The effect of shunting the resistance around the 
condenser has had the practical result of adding 
capacity to the resistance. 

A few experiments with a key, battery and gal- 
vanometer will show what the effect would be of 
adding capacity to a signalling circuit. 

As the insulation in a cable acts as a condenser 
dielectric, the choice of an insulation of little ca- 
pacity is necessary (see Table IV.) 

One of the tests of a cable is to determine its 
capacity, the other two main tests are as to its 
insulation and the conductivity of the conductor 
or core. The insulation of a cable may be that 
between its core and the earth. Or in the case of 
a multiple cable having several separate cores, the 
insulation between these cores. 

Where cables are armored with iron or steel 
wire, or lead covered, the armor or lead covering 
forms one of the two connections in insulation or 
capacity tests. 

142 



CABLE TESTING. 143 

As in most cases, the cable armor or lead sheath 
is in direct contact with the earth, a connection 
may be had with the latter. But a good contact 
on the armor or sheath is preferable. 

In testing multiple cables, all the cores not being 
tested at the time should be grounded. This is 
done by twisting a bare copper wire around a bared 
end of each conductor and then to the armor or 
sheath. 

If no very delicate instruments are at hand, all 
the cores may be so connected or " bunched " 
omitting the armor connection. An insulation test 
made between the bunch and the ground will give 
the multiple insulation of all the cores. This must 
be less than that of one core as the copper surface 
to the insulation is so much increased. If a large 
deflection results, the cores may be thrown off one 
at a time, watching the deflection. Should it sud- 
denly decrease upon detaching a core from the 
bunch, trouble is evident in that core. 

In preparing core ends for attachment to the 
testing circuit, much care must be exercised. The 
insulation should be removed in a tapering form 
as one would point a lead pencil. 

Absolutely no dampness must exist on the in- 
sulation or current will be conducted from the 
copper to the sheath and destroy the value of the 
test. A good plan is to coat the cable from the 
copper under the binding posts with hot paraffin. 

In testing cables one point will be noticed, the 
deflections on an insulation test will gradually de- 



144 ELECTRICAL INSTRUMENTS. 

crease. This is due to electrification of the cable. 
The insulation apparently improves. 

The deflections are generally read after one min- 
ute's electrification. 

In a multiple cable the actual resistance of each 
conductor must be 'worked out and then all the 
resistances added together. Dividing this by the 
number of conductors will give the average in- 
sulation of the cable. 

In speaking of insulation, the cable length must 
be specified. For instance, if a piece measuring 
1000 feet had an insulation of 1000 megohms, a 
piece only half as long would be 2000 megohms. 
Its insulation surface and therefore its leakage 
surface is only one-half. 

It is customary to reduce the result to the 
average per mile. In the foregoing example, the 
insulation would be 1S9 megohms per mile. 

The rule is to multiply the insulation obtained 
by the length of the cable reduced to miles or 
parts of a mile. 

In the direct deflection method of testing cable 
to be described, the same reading should be given 
whichever terminal of the battery is grounded. 
The negative terminal of the battery to the cable 
conductor shows up a fault in a cable quicker 
than the positive pole for reasons to be given later. 

The temperature at which a test of insulation 
is made is of prime importance. The insulating 
properties of gutta-percha, rubber and compounds 
of similar nature decrease very rapidly with in- 



CABLE TESTING. 



145 



crease of temperature. Taking the case of gutta- 
percha at 50 degrees Fahrenheit as of unit resist- 
ance, at 60 degrees it would be less than one-half, 
at 75 degrees only one-fifth, and at 90 degrees 
only one-twelfth. With other insulations the de- 
crease of resistance may be even greater than this. 
It is customary to test cables at 75 degrees Fahren- 
heit or to correct the readings to that temperature. 
Tables of corrections will be found in most 
large works on testing, such as Kempe's " Hand 
Book of Electrical Testing," or Hoskaier's 
" Electrical Testing of Telegraph Cables. " 




i 



fl 



Fig. 83. 



Telephone Test of Insulation. A simple test of 
cable insulation may be made with a telephone 
receiver and battery as in Fig. S3. 

One wire makes good connection with the cable 
armor or lead covering the other wire being touched 
to the cable core. 

Upon making the first contact with the core a 
click will be heard in the telephone owing to the 
charge of the cable. 



146 



ELECTRICAL INSTRUMENTS. 



But if the connection be kept closed for a minute 
or so the cable will be charged. 

Subsequent tapping will give no click if the in- 
sulation be good. The clicks are of course due to 
current leaking through the insulation. 

Cable Testing for Insulation Resistance. In Fig. 
84 B is a battery, R a reversing key, G a galvano- 
meter, 5 a shunt box, C the cable, E an earth con- 



r~d 



D~i 



H, 



Hill III III 



!L 




Fig. 84. 



nection, L a lead from the cable core to the gal- 
vanometer connection. K is a short circuit key, 
that is, one which can be left closed to short cir- 
cuit and cut out the galvanometer and also be 
opened to cut in the galvanometer. 

Before connecting L to the cable, test the insu- 
lation of L. Connect one end as in the figure to 



CABLE TESTING. 147 

the contact of K, depress one button of R and 
open K, using a shunt according to the sensibility 
of G and strength of B. 

No deflection should appear. 

If any deflection deduct it later from readings 
in testing cable. 

Close short circuit key K\ connect other end of 
L to cable and open K. Any deflection of G will 
be due to earth currents. 

If the deflections so obtained are in the directions 
of deflection when battery is used, deduct same 
from cable readings, if in the reverse direction, add 
them. 

Shunt G with say *-—. shunt, closing K and 

press one button of R. As long as K is closed the 
current all goes into the cable in a similar manner 
to charging a condenser. After ten seconds, open 
K, keeping R still down and a deflection of G will 
ensue. 

If off the scale use a higher shunt or reduce bat- 
tery and repeat charge of cable. 

If too small, use lower shunts or increase battery. 

Repeat the latter test several times, allowing 
cable to charge for one minute, noting readings. 
Then repeat as many more times, but depress other 
button of R, reversing battery and deflection. 

The values of these readings may be worked out 
from the galvanometer constant and an average 
taken to represent the insulation of the cable. 



148 



ELECTRICAL INSTRUMENTS. 



The foregoing cannot necessarily cover every 
minor point, but the main operations are given. 

To sum up: First, obtain the constant of the 
galvanometer; this operation was described else- 
where. Second, test insulation of the leads. 
Third, test for earth currents. Fourth, charge 
cable. Fifth, cut in galvanometer and shunt. 
Sixth, reverse battery and repeat fifth. Seventh, 
strike average of readings. 



H'|'|'|'|'|'|'|'|'ltl'l'|H| r-p^- J 

_ i i 



i 



i i i 




I 



Fig. 85. 



Principle allowances to be made are for: shunt, 
battery, insulation of leads and earth currents. 

A slightly different connection for the above test 
is shown in Fig. 85. 

The cable is connected to the reversing key R, 
and a commutator C permits of reversing the bat- 
tery connections, putting either terminal to ground. 
Or the battery can be cut out entirely and the 
cable tested for earth currents by directly ground- 
ing the connection between R and C 



CABLE TESTING. 149 

Testing Ground Connection. Connect positive 
terminal of battery to ground and negative ter- 
minal through shunted galvanometer to ground. 

Shunt should be so adjusted that deflection is 
not off the scale. If necessary put resistance in 
series with galvanometer. Next short circuit 
ground connection by connecting positive wire to 
ground wire on galvanometer. 

The deflections in each case will show the state 
of ground connection. The better it is the less 
difference there will be between the two deflections. 

Cable Insulation with the Willyoung Set K. The 

general scheme of this test is in Fig. 87. 

The cable is normally grounded by its sheathing or 
armor and one terminal of the battery is grounded. 

The constant of the galvanometer is first calcu- 
lated by setting the Bridge as in Fig. 8G. One cell 

of battery is used and the shunt arm is on -— . 

Adjust the rheostat until a deflection of say 10 
degrees is obtained. 

Multiply the resistance unplugged by the deflec- 
tions and the multiplying power of the shunt. 

In this example let the rheostat be 1800, then 
1800 X 10 X 100 - 1,800,000 ohms. This is the re- 
sistance through which the galvanometer is de- 
flected one degree by one cell of battery. 

The cable is now connected in as in Fig. 87, to 
Z>, and a ground connection made from G r at the 
lower left hand side of the case. 



150 



ELECTRICAL INSTRUMENTS. 



Close the battery key for 10 seconds and then 
close the galvanometer key, the deflections being 
noted. 



\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\y^\N 




\\\\\\\\W\\v 



wwwwwvvA 



Fig. 86. 



The battery has been increased to six cells, 
therefore the constant is to be multiplied by 6 
and divided by the deflection obtained. 



CABLE TESTING. 



151 




Fig. 87. 



152 ELECTRICAL INSTRUMENTS. 

Owing to condenser action of the cable there 
will be a rush of current into the cable at first. 
It will be well to shunt the galvanometer by the 

— — shunt until the charge of the cable is com- 
pleted. 

By using larger battery increased resistances 
may be measured. 

Cable Testing by Loss of Charge. In testing the 
insulation of cable by the loss of charge method, 
the cable is charged and discharged, its capacity 
being compared with that of a standard condenser. 

The cable is then charged for one minute and 
disconnected with its terminal free in the air. This 
is done by using a key similar to the Kcmpe dis- 
charge key before described. 

At the end of one minute, the cable is discharged 
and the reading noted. Comparison of several 
such readings show the ability of a cable to retain 
a charge, i.e., its insulation. 

Formula. Let D be the deflection upon in- 
stantly discharging the cable, d that after one min- 
ute. Then the percentage of loss of charge will be 
D-d 



D 



XlOO 



In many cases where delicate apparatus is not 
at hand the loss of charge method can be used for 
approximate results. 



CABLE TESTING. 153 

The writer has tested multiple conductor sub- 
marine mine cables in New York Harbor with a 
few cells of dry battery and the galvanometer in 
a portable testing set. 

When the direct deflection method failed owing 
to want of sensibility in the galvanometer, the 
difference of deflection made after charge, insulate 
and discharge gave results. 

Capacity Tests. The capacity test of a cable is 
similar to that described in testing condensers. 

A standard condenser is charged and discharged 
and the galvanometer deflections noted. The cable 
is then substituted for the standard condenser and 
the deflections noted. Comparison of the two re- 
sults gives a comparison of the capacity of each. 

The calculations are made as in condenser tests. 

Conductivity Tests. The conductivity of a cable 
conductor may be measured with a Wheatstone 
Bridge as in any similar test for conductivity or 
resistance. 

When both ends are available they are connected 
to the bridge in the usual manner. 

If only one end is at hand it is often possible to 
use a second conductor of the same cable, looping 
the two together at the distant end. The resist- 
ance of one will be then one-half the reading if both 
are similar in diameter, material and length. 

If a ground connection becomes necessary it 



154 ELECTRICAL INSTRUMENTS. 

must be made with the greatest care that it inter- 
poses no undue resistance. 

Locating Faults in Cable. If the grounded fault 
were of practically zero resistance and the ground 
connection between the fault and the battery also 
of zero resistance, the location of a fault would be 
easy. 

It would then only be necessary to measure the 
resistance of the cable core up to the fault from 
both ends of the cable. As the core is of uniform 
resistance, the two results would be in the same 
ratio as the lengths of the cable. 

For example, let the total length of the cable 
be 10,000 feet and its total resistance 21 ohms. 
Measuring from one end A the resistance to the 
fault is 14 ohms. From the other end B, the re- 

2 

sistance is 7 ohms. Then as 14 is —of 21, the 

o 

2 

fault would be — of the total length of the cable 

o 

from A. Or 6666 feet from A and 3333 feet from 
B, neglecting fractions. 

But ground connections or faults rarely fulfil 
the above condition, in fact they usually vary from 
time to time. It becomes necessary to use a test 
such as the Varley or Murray, which disregard the 
resistance of the fault. 

A method of locating a fault as in Fig. 88 may 
be adopted in certain cases. 

A good cable and the faulty one are looped at 



CABLE TESTING. 155 

the distant end D. The resistance of the loop A B 
is then measured by the Bridge method. One x 
terminal of a Bridge being grounded, the other 
terminal is applied to A and the resistance A F is 
measured, and similarly the resistance B F. It is 
evident that both A F and B F include the resist- 
ance of the fault to earth. 

The resistances A F and B F being added to- 
gether, subtracting the resistance of A B will leave 
twice the resistance of F to earth. Twice the re- 
sistance because it has been added in both A F 
and B F. 



A-c 



B-c 



Fig. 88. 

If now the resistance of F be subtracted from 
B F it will give the resistance of B to F. Knowing 
the resistance of the cable per foot the distance 
can be determined. 

Example. — Let AB measure 22 ohms; AF 
equal 26 ohms, and B F 16 ohms. Then 26+16 
= 42, and less 22 = 20 ohms. One-half this, or 
10 ohms, is the resistance of the fault to ground. 
The resistance of B F or 16 ohms less 10 = 6 ohms. 

If the cable conductor resistance were 11 ohms 

per mile, the fault would be — of a mile from B, 
or 2880 feet. 



156 



ELECTRICAL INSTRUMENTS. 



To properly understand the Varley and Murray 
tests consider a Wheatstone Bridge the arms of 
which are equal. The rheostat is replaced by a 
length of cable and the unknown resistance is also 
replaced by a length of cable, both being similar 
in resistance per foot. 

If both lengths are the same, their resistances 
are the same, the Bridge balances and no deflec- 
tion takes place. 

Now shorten one length and add resistance R 
in series with it until the Bridge again balances, 




Fig. 89. 



Fig. 89. The added resistance equals that of the 
piece cut off. 

If the resistance per foot is known, the length 
of the shorter piece can be easily calculated. 

Instead of connecting in the battery by a wire 
as shown, use earth connections at each point. 

The only result will be to interpose a resistance 
in the battery circuit. It will not affect the bal- 
ance but only reduce the effective power of the 
battery. This may be overcome by enlarging the 
battery. 



CABLE TESTING. 



157 



Before making the test the copper core resist- 
ance of the loop between M N must be known 
either by calculation or measurement. 

The latter is known as the Varley test, a more 
complete description is as follows: 

The Varley Test. In this test for a fault or 
earth connection the Bridge is connected as in Fig. 
90. The battery is grounded and its circuit is 
completed by the ground at the fault F. The faulty 




Fig. 90. 



cable is looped or connected at its distant end D 
with a good cable. 

To obtain a balance it is clear that the rheostat 
r must be adjusted until its resistance added to 
that of the cable core between M and F together 
equal the resistance of the core from F to N. 

It is also clear that the resistance of the ground 
at F only reduces the energy of the battery current. 

The test is therefore independent of the amount 
of core exposed to the ground at F. 



158 



ELECTRICAL INSTRUMENTS. 



Formula. Using the letters in Fig. 90, let z be 
the resistance of cable from F to M, Y that of the 
cable from F to N, R the resistance of the entire 
cable loop, F the resistance of the fault, r the re- 
sistance of the rheostat when galvanometer is bal- 
anced. 

Then Y + z = K } and Y+F = r+z+F 
or Y = r + z. 

Z then equals — — . 



The distance x of the fault from M is 
R_ L zL 

z 



L 
= — or x 
X 



K 





B 1 






z 




■O 0"vw£ 




M 






g>f 




F 




R 










| 




N 








--" N 


Y 






\ t 






i 
i 




. 





1---7T7- 

O 



Fig. 91. 



The Murray Test. This is somewhat similar to 
the Varley test but one arm of the Bridge is made 
adjustable. One regular Bridge arm is used, the 
other being replaced by the rheostat giving an 
arm of large adjustment. 

The circuit is given in Fig. 91. B is one Bridge 
arm, R the rheostat now the second Bridge arm, 



CABLE TESTING. 159 

Z the faulty cable between M and the fault F, Y 
the remainder of the faulty cable and also the good 
cable in the loop. 

The rheostat is adjusted until there is no deflec- 
tion. From a previous discussion of the Bridge 
this will be seen to occur when the resistance R 
bears the same proportion to Y as B does to Z. 
Or R : Y : : B : Z. 

Suppose for example only the total length of the 
loop from M to TV is 400 feet. Its resistance is 
40 ohms. A fault occurs at a distance from M 
to be determined. 

Arm B is 100 ohms. R is adjusted and on bal- 
ance reads 300 ohms. R is therefore three times 
B. 

Y must bear the same proportion to R that B 
does to Z; here Y must be three times Z. 

As Y and Z added together equal the whole re- 

3 1 

sistance of the cable, Y will be — and Z — of 

4 4 

this resistance (or length). 

Y therefore equals 30 ohms, and Z 10 ohms. 

3 
Or if the length is required, Y is — , that is, 300 

feet, and Z — or 100 feet. 
4 

Rule. To obtain the length cf Z by means of a 
rule for calculation. The resistance B is to be 
divided by B added to R and the result multiplied 
by tie length of the cable loop or L. 



163 ELECTRICAL INSTRUMENTS. 



Same example, • x 400 or — x 400 = 

100 feet. 

Formula. R : Y : : B : Z, or — =— . 

Z+y = L, then Z = L- Y. 
x or distance of F from M = 

x L. 



£ + K 



Murray Test with Aone Set. The layout of the 
Willyoung testing set for the Murray test is in 
Fig. 92. 

Formula. Let r be resistance of loop, Y be 
distance of fault from Z), A resistance unplugged 
in Bridge arm ^4, R resistance unplugged in rheo- 
stat. 

™ R Y v Rr 

Then -: 5 = — , Y = 



i4 + /? r ' A + R' 

In fault testing the negative pole of the battery 
is preferably connected to the cable. If the posi- 
tive pole be applied and there be a bared spot in 
the insulation, this bared spot will be acted on by 
the current, and apparently improved, but the 
chloride of copper formed is at the expense of the 
copper core. 

Locating Fault by Unreeling Cable. In this 
method two insulated reels are used, or an insulated 
tank, a galvanometer, battery, etc. 



CABLE TESTING. 



161 




Fig. 92. 



162 ELECTRICAL INSTRUMENTS. 

One end of the cable conductor is connected to a 
galvanometer the second terminal of the latter 
being grounded through a battery. 

The cable is wound from one reel to the other, 
the slack passing through the water in the tank. 
As soon as a fault reaches the w T ater, the galvano- 
meter will be deflected by the current flowing 
through the circuit including galvanometer, bat- 
tery, ground fault and cable. 

If an insulated tank i* used, the ground connec- 
tion of battery and galvanometer is made to the 
water in it. 



CHAPTER XI. 
Testing With Voltmeter. 

Testing with Voltmeter. The adaptability of a 
good portable voltmeter to everyday testing is 
surprising to those who have not studied it. 

There are few tests outside of high resistance 
tests in cable insulation work that cannot be per- 
formed. And even the latter are possible with a 
sufficiently low reading voltmeter and a large bat- 
tery. 

A voltmeter will do everything that a galvano- 
meter of the same sensibility will, and a great deal 
more. 

The most useful instrument would be a double 
scale portable having one total scale reading one 
hundredth that of the other. This together with 50 
cells of small but good dry battery and a few stan- 
dard resistance coils would make a testing set of 
vast utility. 

Testing Resistance with Voltmeter. In Fig. 93 
is shown the direct deflection method of testing 
resistance R with a voltmeter V. 5 is a switch 
and B a battery. 

163 



164 



ELECTRICAL INSTRUMENTS. 



The e.m.f. of B is first noted by moving the 
switch arm to the left. The arm is then moved to 
the right, and the e.m.f. through R noted. The 
first deflection showed the deflection of V caused 
by the battery B through the resistance of V . The 
second deflection was through the resistances of 
R and V. If V was 10,000 ohms and the first de- 
flection 10 degrees, only one degree of deflection 
would be obtained through ten times 10,000 or 





Ap/ 






J_ 


O w 


R 


■ 1 1 




1 


£ 


AMAAMAn 


— 


"A 


A 


(vvvvVVVv 


B 


\~) 







, \. \*) J 







| 









Fig. 0.°,. 



100,000 ohms, which latter is the constant for V. 
Let the second deflection be 5 degrees. The re- 
sistance of R is therefore '- or 20,000 ohms, 

5 

but the resistance of V must be deducted. 
20,000- 10,000 = 10,000 ohms as the resistance of 
R. 

The constant for the voltmeter may be calcu- 
lated as above, or a formula may be used for the 
entire calculation. 



TESTING WITH VOLTMETER. 



165 



Formula. In the formula let V be the first de- 
flection, V 1 the second deflection, and r be the re- 
sistance of the voltmeter, 



Then 



V xr 

~v7 



— r = x or the resistance of R. 



For measuring low resistances a low reading 
voltmeter or a millivoltmeter is preferable if a bat- 
tery or low e.m.f. be used. 

In many cases the current in a power circuit 
may be used instead of an extra battery. 




Fig. 94. 



Insulation of Generator with Voltmeter. In test- 
ing the insulation of a generator in operation, con- 
nection is made as in Fig. 94 where V is the volt- 
meter, 5 the shaft or part of the framework, and 
B B the brushes. 

The generator furnishes its own testing current. 
The e.m.f. across B B is first noted, then the e.mf.. 
from B to 5 and the last formula applied. 



166 ELECTRICAL INSTRUMENTS. 

Care must be taken that the voltmeter is not 
affected by the generator and that ajl connections 
and contacts are good. 

Insulation Resistance of Armature. A test of 
the insulation resistance of the entire armature 
may be made with the armature- removed or dis- 
connected from the fields. 

A bare copper wire is wound tightly around the 
commutator so as to touch all segments. The 
testing circuit is between the shaft and this copper 
wire. This will give the combined leakage paths 
between all coils and segments to the frame. 

The copper wire ensures a better contact with 
all segments. Where the armature coils are in 
series with each other and tapped to the com- 
mutator, no lower insulation will be noted than 
between one segment and the shaft. That is, pro- 
viding all armature coil connections are good. The 
voltmeter should be in circuit with a tap from the 
mains as in the test for insulation when generator 
is running. 

If the armature is in its place and all field and 
brush connections complete, the test will show the 
entire insulation of the machine. Brushes must 
in the latter case rest on the commutator. 

Testing Electric Light Wiring. A short circuit 
on an electric light system generally indicates itself. 
On the other hand, it may be desired to test the 
mains for such before cutting in the dynamos. 



TESTING WITH VOLTMETER. 167 

Testing an extensive system of wiring for short 
circuits from a main switchboard is a difficult task 
without having an idea of the lamps, etc., connected 
to it. 

If there were 1000 lamps in multiple between 
the leads, the resistance might be as low as one- 
fifth of an ohm. 

Such a low resistance between two leads would 
be a veritable short circuit if it w T as due to defective 
insulation. If it is practicable to turn off all 
lamps first, the matter does not then present much 
difficulty. 

A simple test on a circuit of say 110 volts w T ould 
be to use an incandescent lamp of the same volt- 
age. 

This lamp would be connected between one side 
of the main being tested and a source of e.m.f., 
perhaps a dynamo. A wire is run from the other 
side of the circuit to the other terminal of the 
dynamo. A short circuit on the main would 
cause the lamp to light up. Of course the dynamo 
is to be disconnected from the main by its switch. 

If a voltmeter be used instead of the lamp, the 
actual resistance is determined by simple calcula- 
tion. 

The e.m.f. of the dynamo is to be multiplied 
by the resistance of the voltmeter and the result 
divided by the e.m.f. observed when in series with 
the circuit being tested. Deducting the resistance 
of the voltmeter the answer is the actual resistance 
between the two sides of the main. 



168 ELECTRICAL INSTRUMENTS'. 

In either method, the actual location of the 
trouble is assisted by opening the branch circuit 
switches on the board one at a time. When the 
faulty circuit is reached upon opening its switch, 
the lamp goes out or the voltmeter reading de- 
creases. 

The practice of connecting pilot lamps across 
the circuit at motors and other apparatus is liable 
to cause perplexity. 

A magneto bell would ring easily through one 
lamp. It would be troublesome to measure low 
resistances such as one lamp on the voltmeter, as 
the change of deflection would be slight. The 
lamp test would therefore be preferable for a short 
circuit. 

A short circuit in a branch leading from a cut 
out may be hunted for after replacing one of the 
fuses by a lamp. The lamp will burn until the 
short circuit is removed. A lamp twisted loose 
from its base, or a faulty socket is best found by 
this method. 

If the circuit be cut between the fault and the 
cut-out, the test lamp in the cut-out will cease to 
burn. 

Measuring Switchboard or Line Insulation. This 
method has been before described in the direct 
deflection tests, except that here one terminal of 
the voltmeter is grounded. 

A brief summary of the test between a switch- 
board bus and the ground will be given. 



TESTING WITH VOLTMETER. 169 

Take reading of total e.m.f. of circuit. 

Take reading between bus and a ground con- 
nection. 

Multiply resistance of voltmeter by e.m.f. of 
circuit, divide result by e.m.f. of bus to ground, 
and deduct resistance of voltmeter. 

Answer will be ohms to ground of opposite bus 
to one used in calculation. 

Should both sides of the circuit be grounded the 
calculations become vastly more complicated. 

When testing one side, the voltmeter will be in 
multiple with the fault from the side to which it 
is connected, and vice versa. 

And the two ground connections will form a con- 
nection between the mains of a resistance equal 
to the resistance of each ground added together. 

If it were possible to disconnect one main from 
all connection with the other main, the test would 
be simple. The insulation to ground of each main 
would be ascertained while the other main was 
disconnected. But this is not often practicable. 

Another method would be to disconnect both 
main wires from the bus-bars or from the current 
source, strap them together with a piece of bare 
copper wire, and measure the insulation to ground 
as if they were one wire. 

This would give the joint insulation-resistance 
of the main to ground which after all is what is 
needed most in practice. 

Formula. Let V equal e.m.f. of circuit, V x e.m.f. 



170 ELECTRICAL INSTRUMENTS. 

between main to ground, R resistance of insulation 
to be ascertained, r resistance of voltmeter. 

Then R = — — r. 

In using the voltmeter for measuring resistances 
there is another method of working which gives 
the same result. 

First deduct the second reading on voltmeter 
from the first reading. Then divide this result by 
the second reading and multiply by resistance of 
the voltmeter. 

Example. Let e.m.f. of testing circuit or that 
between mains be 110 volts, resistance of volt- 
meter 15,000 ohms, reading through resistance 
ground, etc., be 10 volts. Then 110-10 = 100, 

~j?X 15,000 = 150,000 ohms. 

Working this by the first method, — 

= 165,000, deduct 15,000 = 150,000 ohms as before. 

The chief advantage of the first method is that 
a constant can be figured for the particular in- 
strument at a given voltage as described below. 

The formula will be for the second method. 
Let V be e.m.f. or circuit, V\ e.m.f. through re- 
sistance, r resistance of voltmeter then 

~vf~ xr = x 



TESTING WITH VOLTMETER. 



171 



TABLE V. 
Insulation Resistance. 



Vl or Voltage 


V or Voltage of 


V or Voltage of 


through Unknown 


Circuit = 110 


Circuit = 110 


Resistance. 


r or Resistance of 


r or Resistance of 




Voltmeter = 16,500 


Voltmeter = 17,000. 


1 


1,798.500 


1,853.000 


2 


891,000 


918,000 


3 


588,500 


606,333 


4 


432.750 


450,500 


5 


340,500 


357,000 


10 


105,0^0 


170,000 


20 


74,250 


76,500 


30 


44,000 


45,333 


40 


28,875 


29,750 


50 


19,800 


20,400 


100 


1,650 


1,700 


110 









Resistance Table for Voltmeter Tests. Table V. 
is computed for ready reference in insulation or 
resistance tests with a Weston voltmeter or other 
instrument of high resistance. 

The first column gives the reading through the 
resistance, the second and third columns the actual 
resistance of the insulation. 

Both the latter columns are figured for a line 
e.m.f. of 110 volts and for an instrument resistance 
of 16,500 and 17,000 ohms respectively. 

Where a number of readings have to be made it 
is handy to figure a constant, that is, multiply the 
resistance of the voltmeter by the e.m.f. of the 
circuit. 

The result or constant then can be used by sim- 
ply dividing it by the e.m.f. read with insulation 
in series and subtracting the resistance of the volt- 
meter. 



172 ELECTRICAL INSTRUMENTS. 

Resistance of Voltmeter. This may be deter- 
mined by one of the tests given to find resistance 
of a galvanometer. The one-half deflection is 
perhaps the simplest. 

Connect voltmeter to a steady e.m.f. and read 
deflection. 

Then add resistance in series with voltmeter 
until deflection is reduced to one-half. The added 
resistance will equal resistance of voltmeter. 

This is clearly so because the one-half deflection 
is due to one-half current, and one-half current 
is due to doubling the resistance of voltmeter. 
In order to double the voltmeter resistance a re- 
sistance has been added which is equal to it. 

Formula. Let V represent first deflection, V x 
the one-half deflection, R resistance of voltmeter, 
r resistance added. 

Then V t = ^ when r = R. 

Testing e.m.f. around Armature. There are two 
methods of testing the distribution of e.m.f. around 
the armature and determining the equality of the 
magnetic field. 

One is as shown in Fig. 95. H is an insulating 
handle carrying two carbon brushes A B from 
which run wires C D for the voltmeter connection. 

A B are set just far enough to span the insula- 
tion between two commutator segments. 

By making contact on the commutator and 



TESTING WITH VOLTMETER. 



173 



gradually moving H in the direction of rotation, 
the e.m.f. is tested at different points in the arma- 
ture under the influence of various parts of the field. 




Fig. 95. 




Fig. 96. 



These e.m.fs. being plotted on a chart give a 
graphic record of the distribution of field flux. 

Another method is Fig. 96, where one movable 
brush is used, the other voltmeter connection 
being on one of the regular working brushes. 



174 



ELECTRICAL INSTRUMENTS. 



This shows a gradual rise or fall of e.m.f. until 
the total e.m.f. of the generator is reached, but 
not so readily the actual e.m.f. of any one coil at 
a time. 

Measuring Drop across a Lamp. To measure the 
drop or voltage expended in a lamp while burning 
the connections should be as in Fig. 97. L is the 
lamp, V the voltmeter and an ammeter A is shown 




Fig. 97. 

so that the current consumption can also be fig- 
ured. The ammeter also is necessary if the resist- 
ance of L is to be determined. 

The drop across L will be directly indicated by 
V. The current shown by A multiplied by the 
e.m.f. at L will give the watts expended in L. 



Lamp Current Efficiency Test with Voltmeter. 

The amount of current taken by an incandescent 



TESTING WITH VOLTMETER. 175 

lamp on a given voltage depends upon the resist- 
ance of its filament. 

Comparison of the hot resistance of one lamp 
with that of a standard lamp will therefore show 
the current efficiency of the first lamp. 

If a low reading ammeter be available the cur- 
rent measurement may be made direct. But such 
an ammeter is not always at hand. 

The drop across each lamp is to be measured 
with the voltmeter as in Fig. 97. 

The e.m.f. indicated in each case will be propor- 
tional to the resistance of the lamp. And the cur- 
rent consumption will be in proportion to the re- 
sistance of each. The higher the e.m.f. across the 
lamp terminals, the higher will be its resistance 
and the lower its current consumption. 

The e.m.f. of the testing circuit must be the 
same in both operations. 

This test does not show the true efficiency of 
the lamp as no data is obtained of the relative 
candle powers. 

And the resistance of L can be computed by 
dividing the current reading of A by the e.m.f. 



reading of V or R = l-zr-f- 



Suppose V reads 100 volts and A .5 ampere. 
Then 100 X. 5 = 50 watts, and 100 -^ .5 = 200 
ohms. 

Testing a High Voltage with a Low Reading Volt- 
meter. This is accomplished by the fall of poten- 



176 ELECTRICAL INSTRUMENTS. 

tial method. Suppose it is desired to measure the 
e.m.f. across a circuit of about 550 volts with a 
voltmeter reading to 150 volts. 

Connect five 110 volt lamps L in series across 
the circuit, Fig. 98. 

Then measure the e.m.f. between A B, B C, C D, 
D E and E F and the sum of these e.m.fs. will 
equal that between A F. 

Care must be taken that the connections of the 
voltmeter are made so as to include all parts of 
the circuit in turn. For instance, when shifting 
from A B to B C, the terminal from ,4 must be 



+ 
-9 



i 

: B 



C 'D 

Fig. 98. 



•F 



placed exactly at B where the other terminal of 
the voltmeter was. Or part of the e.m.f. in the 
wire or connection may be unmeasured, if this be 
neglected. 

The lamps must all be kept burning during this 
test, or the voltmeter may be injured. 

Temperature and Resistance. In testing coils of 
wire with their normal current flowing it will be 
found that the resistance will gradually increase, 
due to the current flowing in them. This increase 
is about .004 ohm per degree Centigrade for each 



TESTING WITH VOLTMETER. 



177 



ohm of the coil (see Table VI). This is of very- 
great importance in generator and motor tests. 
It would reduce the e.m.f. of a shunt generator 
owing to decreased field current. And it would 
speed up a shunt motor by reason of decreased 
field and decreased counter e.m.f. 



TABLE VI. 

Table Showing Increase in Resistance of Pure Copper Wire for Rise of 
Temperature above a Given Point. 





Increase in 




Increase in 


Centigrade 


ohms per ohm 


Centigrade 


ohms per ohm 


1 


.00389 


21 


.08169 


2 


.00778 


22 


.08558 


3 


.01167 


23 


.08947 


4 


.01556 


24 


.09338 


5 


.01945 


25 


.09725 


6 


.02334 


26 


.10102 


7 


.02723 


27 


. 10503 


8 


.03112 


28 


. 10894 


9 


.03501 


29 


.11281 


10 


.03890 


30 


.11680 


11 


.04279 


31 


. 12059 


12 


.04669 


32 


.12450 


13 


.05051 


33 


. 12837 


14 


.05447 


34 


.13226 


15 


.05835 


35 


.13615 


16 


.06225 


36 


.14061 


17 


.06613 


37 


.14593 


18 


.07030 


38 


.14782 


19 


.07391 


39 


.15171 


20 


.07780 


40 


.15575 



This table is computed from an average between five authorities, 
The increase in ohms per ohm shows the fraction of an ohm which a wire 
one ohm in resistance would increase if its temperature were raised as 
per column of temperature. The coefficient .00389 is used but for 
ready calculation; .004 is used in the formulas, there is no absolute 
standard. To reduce Centigrade to Fahrenheit, multiply by 9, divide 
by 5, and add 32 to result. 



178 ELECTRICAL INSTRUMENTS. 

All tests of generator and motor should record 
the temperature both of the air and machine. 

The thermometer used for the air test should 
lie a few feet from the end of the shaft so as not 
to be influenced by air currents from a revolving 
armature. 

And of course it should not be near any hot 
pipes or engine cylinders. 

Temperature tests of field coils and armatures 
may be made with a thermometer laid on the part 
under test and covered with cotton waste. But 
such tests do not show heat inside the coils and 
are unreliable. A more reliable method is by tak- 
ing advantage of the rise of resistance in the coil 
due to temperature rise. 

The resistance of the coil cold may be measured 
and recorded. Its resistance is then measured 
from time to time and the temperature calculated. 

For example, let the cold resistance of a shunt 
field coil be 120 ohms. One-half hour later it is 
135 ohms. 

This is a rise of 15 ohms, as it is the rise on 120 

ohms, the temperature will be — ^- — : ■* .004 

120 ohms 

or 32° C. 

A simple way to find the temperature rise is to 
divide the difference between the cold resistance 
and the hot resistance by the total resistance cold. 
Then find the nearest figure in the resistance col- 
umn of Table VI. Against this will be the tem- 
perature. In this example 



TESTING WITH VOLTMETER. 179 

135-120 = 15,^ = .125. In Table nearest is 

.124 or 32° C. 

A third method is divide the difference between 
the two resistances by the cold resistance and mul- 
tiply by a constant 252 for the Centigrade rise. 

Same example. .125x252 = 32. 

The temperature test by resistance shows the 
average temperature of the coils. 

For test of the temperature at specific points, a 
special device may be prepared. 

A strip of mica about two inches square is wound 
with 200 turns of No. 36 silk covered copper wire. 

The strip is then fixed to a convenient handle 
if desired. This coil may be inserted in crevices 
between the coils of the armature, etc., its resist- 
ance being measured from time to time w T ill give 
the temperature by computation as before. 

Formula. Let R be the cold resistance, H be 
the hot resistance, t C the temperature in Centi- 
grade degrees, t F the temperature in Fahrenheit 
degrees. 

Then / C = H ~ R X 252. 
K 

And t F = ^-^ X 423. 
K 

Testing for a Break in a Coil. Testing for a 
break in a field coil of a dynamo or motor may be 
done by simply connecting a bell and battery in 



180 



ELECTRICAL INSTRUMENTS. 



series with the coil, Fig. 99, and trying if the bell 

rings. If it rings the circuit of course is complete. 

But some field coils are of high resistance in 

which case a voltmeter, Fi^. 100, is substituted for 




Fig. 99. 

the bell. A lead may be taken from an electric 
light circuit to replace the battery. And if no 
definite readings are desired an incandescent lamp 
may replace the voltmeter. 




Fig 100. 



But the voltmeter method will show the extent 
of any fault in the circuit or the resistance of the 
latter. The resistance is calculated from the de- 
flections by the method given elsewhere, 



TESTING WITH VOLTMETER. 



181 



Testing the Resistance of Coils. If a number of 
coils are to be compared, as in a motor or dynamo, 
they may be tested without disconnecting them 
other than from the brushes, Fig. 101. 

Pass current through the coils as they are in 
series, using a rheostat R if necessary, a bank of 
lamps will answer. 

Measure the e.m.f. across the terminals of each 
coil, AB, CD, EF and G H, and compare the 
readings; they should all be equal. If not, the 




Fig. 101. 



low reading coils are short circuited unless the high 
reading coils are faulty or open. 

The actual resistance of each coil can be computed 
if an ammeter .4 M be put in series to give the 
current value. 

By ohms law the resistance will equal the e.m.f. 

E 
divided by the current or R = -y-. For example, 



182 ELECTRICAL INSTRUMENTS. 

if the readings show one ampere and 15 volts, the 

15 

resistance will be — or 15 ohms. 



Testing Armature Coils. Armature coils being 
of low resistance can best be tested by the fall of 
potential method. 

Current from an electric light circuit is passed 
through a number of coils by connecting the leads 
to segments situated some distance apart. The 
exact distance will depend upon the instruments 
available whether low or high reading. The e.m.f. 
is then read between segments or from one segment 
to the others in succession. 

A rheostat or lamp bank should be inserted in 
series with the current leads. 

If the e.m.f. between segments is uniform the 
coils and connections are good. 

If a jump occurs at a segment, the voltmeter 
needle moving a greater distance than usual, a 
poor connection is probably the cause. 

Testing Armature Resistance. The connections 
for this test are in Fig. 102. 5 is a switch connected 
to a source of current. A an ammeter, V a milli- 
voltmeter, L one or more incandescent lamps for 
resistance purposes to control testing current. 
Field wires of a shunt machine are disconnected. 
Current from 5 passes through the armature coils, 
ammeter, and resistance lamps. 



TESTING WITH VOLTMETER. 



183 



The connections may be made with copper wire 
making good contact on commutator segments by 
inserting them under brushes SB of opposite 
polarity. 

The armature is stationary during this test. 

The contact must be good, a strip of copper sheet 
laid flat on the segments is better than a mere wire. 
The copper strips should be held firmly down on 
the segments. 




Fig. 102. 



Readings are taken of the e.m.f. between the 
segments to which the connections of the current 
circuit are made. 

Readings are also taken of the current indicated 
by the ammeter. 

Then by ohms law, the resistance equals the 
e.m.f. divided by the current. 

For example, let the current I be one ampere, 
the e.m.f. one-tenth of one volt, then one-tenth 



184 



ELECTRICAL INSTRUMENTS. 



divided by one equals one-tenth, or one-tenth of 
an ohm. 

If the armature resistance is so low that a very 
small reading is given on the millivoltmeter, in- 
crease the current flow by adding more lamps in 
multiple. 

The importance of good voltmeter contact at 
the commutator segments will be seen if a number 
of tests are made. Improve the contact in each 
case until no increase of deflection is noted on 




Fig. 103. 



voltmeter. The larger the current through the 
coils the easier the test. 

A form of test where an ammeter and battery 
are used is given in Fig. 103. 

The milammeter A is adjusted by means of a 
shunt 5 to give a large deflection when the ter- 
minal leads are short circuited. Its leads are then 
touched on adjacent segments of the commutator. 

The deflections as compared with one another 
give the comparisons of coil resistance. 



"TESTING WITH VOLTMETER. 



185 



If a coil is open or a connection broken, the 
ammeter will indicate low reading when spanning 
its segments. 

If one coil is partly open, the high resistance 
at the fault will give a low reading on the ammeter. 

If a coil is vshort circuited, the deflection will be 
greater than that across a good coil. It will be 
equal of course to the deflection obtained by touch- 
ing the ammeter leads together. 



BC D 


k 




A 




o o 


1 



Fig. 104. 



Voltmeter Test of Current in Circuit. It is often 
desired to know the current flowing in a lead 
when it is impracticable to cut the lead and insert 
an ammeter. 

A very close approximation may be made with 
a low reading voltmeter by measuring the drop 
along a part of the wire. 

The voltmeter is connected between two bared 
points on the cable as in Fig. 104. The resistance 
of the cable between these points is read from a 
table of copper wire resistance. The connections 
must be made with care and the leads to the in- 



186 ELECTRICAL INSTRUMENTS. 

strument be large enough to interpose no appre- 
ciable resistance. 

Take the case of a cable No. B. & S. The 
resistance per thousand feet is .096 ohm. 

The low reading or millivoltmeter is connected 
across ten feet and a reading observed of 14.4 
hundredths of a volt. 

As the resistance of ten feet is .00096 ohm, by 

.144 

ohms law = = 150 amperes. 

This low reading is perfectly possibly on the 
double scale instruments mentioned at the begin- 
ning of this chapter. 

Another method would be to measure the e.m.f. 
across the line at two points. The first would be 
nearest to the generator or switchboard, the sec- 
ond at the point where the cable branched off to 
smaller wires. 

The difference between the two readings would 
be due to the loss in the line and the loss beyond 
the point of second reading. 

The loss in the line divided by the computed 
resistance of the line would give the current flowing. 

The line resistance is of course its length multi- 
plied by its resistance per thousand feet. The 
actual length of wire is to be used, t.iat is twice 
the distance between the two points. 

Example. Cable 250 feet between points, in 
all 500 feet No. B. & S. Resistance .048 ohm. 
E.m.f. at first point 115 volts at second point 110 
volts; loss in line thus 5 volts. 



TESTING WITH VOLTMETER. 187 

E 5 5 

— = — - - or neglecting fraction -— = 100 am- 
R .048 fe .0o 

peres. 

The readings must be made as near simultane- 
ously as possible, two adjusted voltmeters will be 
preferable although not absolutely necessary. 



Corrections in Resistance by " Drop " Methods. 

If great accuracy is desired the drop through the 
voltmeter may be allowed for. The current in 
the voltmeter will be found by dividing the e.m.f. 
indicated by the voltmeter resistance. This cur- 
rent is to be deducted from the ammeter readings. 

In armatures where the coils are in series but 
tapped off at commutator segments, the path 
through the armature coils will be double between 
brushes. One path is through coils on top section 
and another path through bottom section coils. 

If a coil were open in its windings, current would 
still flow to the testing instruments by the second 
path. But a jump of the needle would be shown 
on the voltmeter when spanning the break. 

If a connection between coil and segment were 
broken, the ammeter would not show any less 
current except at the instant when the faulty seg- 
ment was under the brush or connection. 

Plotting Curves. A graphical method of record- 
ing tests is by plotting curves on sectional paper. 
In Fig. 105 are shown the readings of e.m.f. 



188 



ELECTRICAL INSTRUMENTS. 



taken half way around the commutator of a bipolar 
generator. 

The figures at the left hand side of the chart 
are the voltage readings. The figures along the 
bottom of the chart represent the segment number. 

The readings along the horizontal lines are the 
abscissae, those up the perpendicular lines, the 
ordinates. 















































































































150 














































































































































































































































































100 






















































































/ 




















































/ 




















































/ 


















































f 


/ 


























50 
























<■ 


/ 
















































» 


























































































































































s 




















































































































































5 10 15 

Fig. 105. 



20 



25 



In making such a chart, the figures representing 
the voltage and segment numbers are first marked. 

Then as a reading is taken a dot is made at the 
intersection of the lines representing the e.m.f. 
and the segment. When the total number of 
readings is made, the dots are connected by a line 
running through them. 



TESTING WITH VOLTMETER. 189 

For example, the first reading between brush and 
segment 4 is 5 volts, a dot is made on the vertical 
line corresponding to No. 4 segment, and at the 
point where a horizontal line from 5 volts would 
cut it. 

A second reading of 10 volts on segment 5 is 
similarly marked, and another 15 volts at segment 
6, and so on. 

If the divisions represented by the abscissae and 
ordinates are not close enough, computation may 
be made with a pair of dividers. But cross section 
paper for such charts is made with 8 and 10 
divisions per inch. 



APPENDIX. 



Testing Telegraph Wires and Cables 
and 
Locating Faults in Telegraph and Tele- 
phone Wires and Cables. 

BY 

JESSE HARGRAVE 

Asst. Electrical Engineer Postal-Tele graph-Cable Company. 

With 28 new Diagrams. 



191 



CHAPTER XII. 
Testing Telegraph Wires and Cables. 

Early Morning Tests. All wires leading from 
each terminal station are tested out regularly every 
morning not later than seven o'clock for continuity, 
earth contacts, or "grounds," and crosses. The 
wire chief simply calls up the distant chief on some 
wire known to be intact and tries out first one wire 
and then another until all have been tested. Fail- 
ing to get the distant station on any particular wire, 
if it is found that the testing relay fails to open 
while the distant end stands open, then the wire is 
in contact with the earth or else mixed with some 
other wire. If the testing relay remains open while 
the distant end is to opposite battery pole or to the 
ground, then the wire is open. If the wire be mixed 
with some other wire it is usually quickly found out 
by noticing a disturbance between the two or the 
development of some other symptom with which 
the testing chief very soon becomes familiar. 
Fig. 106 shows the testing relay standing closed to 
an intermediate ground while the distant end is 
open. 

193 



194 ELECTRICAL INSTRUMENTS. 

Fig. 107 shows the testing relay standing open to 
a break while the distant end is grounded. 

When a wire has been found to be grounded or 
open, each intermediate testing station, beginning 
at the distant end and proceeding in regular order, 




i 



Fig. 106. 



jr~\jDp^ 



Fig. 107. 

is told to open or ground the wire until the location 
of the fault is definitely determined between stations. 
In the case of crossed wires, the distant station 
is told to open both wires, and a battery is then 
applied to one, and the other is grounded through 
the testing relay. If the battery on one wire closes 



TESTING TELEGRAPH WIRES AND CABLES. 195 

the relay on the other wire, it is quite obvious that 
it does so through a cross between the testing station 
and the distant station's opening. Successive sta- 
tions are then called up in regular order and told to 
open the wires until the fault is localized. Fig. 108 
shows the testing relay closed through the cross. 



i 






cam 

] fm^ 



Fig. 108. 



Wrecks. It frequently happens that all wires 
become mixed by reason of a tree falling across the 
line or other cause. Such conditions are known 
as "wrecks." It usually takes some little time for 
the wire chief to test the wires out and determine 
definitely just what conditions exist at the wreck — 
that is, what wires are crossed with each other, 
which are open or grounded, etc. The writer has 
found that the test outlined in Fig. 109 to be the 
quickest and surest way of testing out such trouble. 
Each wire in turn is connected to the test relay whose 
other terminal is connected to ground. All other 
wires are then opened and a moderately strong bat- 
tery is placed to one at a time. Those that are 



196 



ELECTRICAL INSTRUMENTS. 



crossed with the wire under test will close the relay 
the moment the battery is applied to them, provided 
the wires are not "dead grounded. " The spring of 






D 



H>' 



Fig. 109. 



AUTOMATIC SIGNAL WHEN WIRES COME O. K. 




Fig. 110. 



relay should be left reasonably slack while this test 
is being made. 

When an "opening" or "ground" is located on a 
wire by any of the methods described above, it is 



TESTING TELEGRAPH WIRES AND CABLES. 197 

customary for the wire chief to order the lineman 
upon whose section the fault is located to hunt for 
it, giving all the information possible which will 
assist him in finding and removing it with the least 
possible delay. It is then very important that the 
wire chief know the instant that the wire is cleared 
of the trouble, and in order to accomplish this result 
the arrangement outlined in Fig. 110 is much used 
by one of the large telegraph companies. If the 
wire be "grounded" the first station beyond F is 
instructed to leave it open. Relay R is then cut 
into the wire as shown and switches S and 5 1 thrown 
to the left, which throws bell B into circuit on back 
contact of relay R. So long as the wire remains 
grounded at F relay R remains closed and its arma- 
ture rests on its front contact, thus leaving the bell 
circuit open. The moment the lineman removes 
the ground, however, relay R opens and rings bell 
through its back armature contact. The wire chief 
then reverses both switches, throwing sounder S 2 
into circuit on front contact of R, and proceeds 
to test out the wire in the usual manner and assign 
it to a circuit. 

Should the wire be open instead of grounded, 
the distant station is instructed to keep it to his 
ground, switch 5 is thrown to the right and switch 
S 1 to the left. It will then be seen that the moment 
the lineman closes the break R will close to the 
distant station's ground, thus ringing bell B through 
its front contact. Both switches are then placed 



198 ELECTRICAL INSTRUMENTS. 

to the right, and the chief proceeds to test out the 
wire in the usual manner. 

Locating Grounds by Wheatstone Bridge Meas- 
urements. When a ground has been located on a 
wire by the method described above it is often de- 
sirable to make a closer and more exact location 
of it by electrical measurement in order that every 
facility may be afforded the lineman in finding and 
clearing the trouble in the shortest possible time. 
In making this test the Varley loop measurement 
described in Chapter X is usually employed. A 
good wire is looped with the faulty one at the next 
station beyond the fault, as shown in Fig. 90. The 
mode of procedure is then the same as described 
under head of 'The Varley Test" on pages 157-8. 
The values of the ratio arms A and B are usually 
made equal and should be that value which more 
nearly approaches the resistance of the loop under 
measurement. 

For example, if the looped wires measure 400 
ohms, then 100 ohms should go in arms A and B, 
whereas if loop measures GOO ohms, then 1000 ohms 
should be used in the arms of bridge. Great care 
should be taken in this, as in all other bridge tests, 
to insure a proper battery strength. If dynamo 
current be used, a reducing rheostat should invari- 
ably be placed in circuit and sufficient resistance 
should be cut in as to obviate any danger of over- 
heating the coils of the bridge. It is sometimes 



TESTING TELEGRAPH WIRES AND CABLES. 199 

advisable to cut a milli-ammeter in circuit with the 
battery so that a check can be kept on the current 
entering bridge. 

Measurement for Cresses using Varley Test. If a 

good, clear wire is available it should be looped 




Fig. 111. 




F 

T 



Fig. 112. 



with one of the crossed wires at the first convenient 
station beyond the fault and a measurement made 
of the loop, as in Fig. 111. 

Another measurement is then made as in Fig. 
112. 



200 ELECTRICAL INSTRUMENTS. 

It will be seen that the battery instead of being 
led to the loop through the distant fault via 
ground, as is shown in Fig. 90, is carried via the 
crossed wire to point F. It sometimes happens that 
the end of the second crossed wire is not available 
for joining on to battery, and in this case the distant 
station at which the wires have been looped is told 
to ground the second crossed wire, and the battery 
is then grounded as in Fig. 113. It should be borne 
in mind that in this case the earth serves the same 
purpose of conducting the battery to the loop via 
the fault as did the second crossed wire in Fig. 112, 
the path of current being shown by the small 
arrows. 




1'1'l'f'i^ ^ j > 



Fig. 113. 
The formula for this test is the same as given 

T E) 

on page 158, or, boiled down, it is simply F= — - — 

where F is resistance to the fault, L is resistance 
of loop as per Fig. Ill, and R is result of measure- 
ment as per Fig. 112 or Fig. 113. The above is, 
of course, assuming that equal values are used in 



TESTING TELEGRAPH WIRES AND CABLES. 201 

arms A and B of bridge, which is almost always the 
case in measurements of overhead wires. The rea- 
son for this is that, as one ohm represents the 
fractional part of a mile of overhead wire, it is not 
necessary, except in unusual cases, to obtain closer 
locations by means of multiplied ratios in arms A 
and B. In No. 9 B. & S. gauge copper wire one 
ohm represents a little less than a quarter of a 
mile, whereas in No. 8 gauge iron wire it represents 
a little less than one-twelfth of a mile. 

When the value of F has been obtained by the 
above formula it is easy to arrive at the distance 
by dividing this result by what the wire measures 
in ohms per mile. 

Although the Varley loop test only is spoken of 
in connection with the foregoing tests for crosses 
and grounds, yet it will readily be seen that the 
Murray test described in Chapter X is applicable. 
The Murray test is preferred by some by reason 
of its simplicity, especially where the two looped 
wires are of the same kind and gauge, in which case 
the initial, or loop, resistance measurement is dis- 
pensed with and the result is figured out directly 
in miles. It should be borne in mind in connection 
with this test, however, that any chance inequality 
in resistance of the two wires or imperfect connec- 
tion or contact at distant stations or elsewhere would 
disturb its correctness. For this reason it is per- 
haps safer to figure the result in ohms and then 
divide by the ohms per mile to get at the distance. 



202 ELECTRICAL INSTRUMENTS. 

Measurement for Crosses Using the Two Crossed 
Wires Only. A study of Figs. 112 and 113 will 
soon convince the reader that the resistance of the 
cross between the two wires can be neglected. So 
long as it furnishes a path for sufficient current to 
enter the loop in order to give a readable galvan- 
ometer deflection on an unbalanced condition of one 
or two ohms, then it does not matter whether its 
resistance be 1 or 1000 ohms so far as affecting the 
result is concerned. It is this fact which makes this 
particular measurement so reliable and to be pre- 
ferred to a simple resistance measurement through 
the cross. 

FIRST MEASUREMENT. 




Fig. 114. 



When a third good wire is not available for this 
measurement, however, a reasonably reliable loca- 
tion of the trouble can be made by tests outlined in 
Figs. 114 and 115. In these tests the resistance of 
the fault is taken into account, and so long as it does 
not change between the two tests it does not alter 
the result. 



TESTING TELEGRAPH WIRES AND CABLES. 203 
SECOND MEASUREMENT. 

L — r 
The simple formula for this test is F— — - — ' 

where F is resistance to fault, L is result of meas- 
urement as per Fig. 114, and r is result of measure- 
ment as per Fig. 115. The explanation of this test 
is that the resistance necessary in rheostat in Fig. 
115 to make a balance simply places point X at the 
theoretical center of the loop formed by this resist- 
ance and the two wires through F. It is obvious, 
then, that to deduct this resistance from the loop 
measurement of Fig. 114 and then halve the result 
gives the resistance to point X. 




If in Fig. 114 we obtain a balance with 220 in R 
and in Fig. 115 a balance is obtained with 50, then 
220 — 50 gives us 170, and this divided by 2 gives 
us 85 as resistance to X. Let us further assume 
that the wires are of the same kind and gauge ; then 
it follows that 85 ohms is also the resistance be- 
tween A and Z, which leaves us 50 ohms for the 
fault. Then 50 + 35 on one side of X equals 50 -J- 85 



204 



ELECTRICAL INSTRUMENTS. 



on the other side, which balances the system and 
proves the result to be correct. 

Fig. 116 outlines another method of locating a 
cross by employing the two crossed wires only. In 
this test it will be seen that the resistance of F is 
neglected, but the resistance between wire a and 
ground at distant station figures in the result and 
cannot be accounted for. "Earth currents," or dif- 
ference in potentials between the earth at points Gr 
and G, would cause considerable error in this meas- 
urement, and for this reason it is not recommended, 
although the writer has at times made use of it 
with more or less success. 




L-|i|i|i|i|i|f-4.gr. 

Fig. 116. 



In arranging for this test the next distant station 
beyond fault F is asked to ground one of the wires, 
and this wire is connected to point L of bridge. 
Arm A of bridge is then shunted out with a piece 
of copper wire or else plugged up, and that ter- 
minal of galvanometer which normally connects 
to point L is detached and connected to the 



TESTING TELEGRAPH WIRES AND CABLES. 205 

second crossed wire &. It will then readily be 
seen that that section of wire a between L and 
F bears the same ratio to the section between 
F and G as arm B bears to the resistance un- 
plugged in R to obtain a balance. It will also be 
seen that a can either be reckoned as having so much 
ohmic resistance or as being so many miles in 
length. For example, assuming that the distance to 
point G is 50 miles, then the distance to F is 

50 X 1000 QQ _, ,_ ., 

— Y^kk — or 33 1/3 miles. 




VM. 



V — icy 156 VOLTS 



Fig. 117. 



Locating a Cross by Voltmeter Test. A simple volt- 
meter measurement for a cross on two wires, but 
one which is not recommended for its reliability, 
is outlined in Figs. 117 and 118. Were it not for 
errors which are likely to arise by reason of earth 
currents this test would be convenient and useful. 
The same objections hold against it, however, as are 
cited against the test described in the foregoing 
paragraph. A voltmeter of very high resistance is 
necessary in this test. 



206 ELECTRICAL INSTRUMENTS. 

If the potential at point L in Fig. 117 is found 
to be 150 volts, then it is obvious that there is a 
gradual drop of 150 volts along wire a until point 
G is reached, where it falls to zero provided G is 
a neutral ground and there is no difference of poten- 
tial between it and the ground at the testing station. 
It then follows that if the potential has fallen from 
150 at L to 100 volts at fault F, as indicated in Fig. 
118, it has dropped through one-third of its value 




Fig. 118. 

between L and G, and if the distance between those 
points be 100 miles it is further obvious that the dis- 
tance between L and F is 33 1/3 miles, or 

150 volts — 100 volts X 100 miles 001 

— =33^ miles. 

150 volts 6 

It will be noted that the accuracy of this test is 

affected by the ratio that the resistance of the wire 

b between F and V M bears to the resistance of 

V M. If, however, the resistance of V M be very 

high, say sixty thousand or more ohms, then the 

error due to this cause would be slight. The test, 

however, is a rough one at best and is not recom- 



TESTING TELEGRAPH WIRES AND CABLES. 207 

mended except where the instruments for the other 
more reliable tests are lacking. 

Insulation Tests — Milliammeter Method. It is 
customary to make periodical tests of the insulation 
resistance of telegraph and telephone wires. Up 
to a few years ago these tests were made with the 
tangent galvanometer almost exclusively, but with 
the advent of direct-reading voltmeters and milli- 
ammeters, quicker and in every way more satis- 
factory methods for making them were found. 



*™EM7 


-200-fcHbES 


— > 


> 


vf 












_^"_ 


r 














■ ~ > 


< 


■=-8 

*— r-l 


f 






T , 


t 


Fig. 119. 





The milliammeter furnishes an accurate and 
ready means of making these measurements, the re- 
sults of which are directly figured out by Ohm's 
law. The simple method of making the tests is 
illustrated in Fig. 119, where the wires are all 
shown open at the distant station. The wire chief 
then quickly inserts his meter in first one and then 
the other, applying an appropriate battery as shown, 



208 ELECTRICAL INSTRUMENTS. 

until he has run through the entire number on that 

particular pole line. It will be noted that all wires 

are grounded except the one under test. This is to 

not only measure the amount of leakage between the 

wire under test and earth, but also any leakage which 

may exist between it and any of the other wires. 

The test is ordinarily made with a double-scale 

milliammeter, employing the most sensitive coil, 

which usually records on the lower scale. This 

lower scale is usually calibrated in divisions of .2 

milliampere (or .0002 ampere). Referring to 

Fig. 119, suppose that with a pressure of 100 volts 

a deflection of 2 divisions on "lower" scale is noted. 

This would denote that a current of four-tenths of 

a milliampere (.0004 ampere) was passing out 

to the "open" wire from 100 volts' pressure. Then, 

R 
by Ohm's law, R — j . hence 100 volts -f- .0004 

ampere = 250,000 ohms, which would represent the 
total insulation resistance of the wire. In consider- 
ing this test the mileage insulation resistance is 
always reckoned, and this is arrived at by multiply- 
ing the length of the wire in miles by the total or 
absolute insulation resistance. Assuming that our 
wire in this instance is 200 miles in length, then 
the insulation per mile would be 50,000,000 ohms, 
or 50 megohms. 

4 

Insulation Test by Voltmeter Method. This test is 
made in the same manner as described in Fig. 119, 



TESTING TELEGRAPH WIRES AND CABLES. 209 

substituting the voltmeter for the milliammeter. The 
total insulation resistance is arrived at by methods de- 
scribed in Chapter XI, and the mileage insulation re- 
sistance is then computed as shown in last paragraph. 
When the presence of an abnormal leakage is 
disclosed by these tests it is customary for the test- 
ing chief to localize the same by means of sectional 
tests ; that is, by having successive intermediate 
stations open the wire for test until the leakage is 
located between two of them. The lineman who has 
charge of that section is then ordered to clear it. 

Conductivity Tests. Conductive resistance tests are 
for the purpose of checking up joints and connec- 
tions and, in the case of iron wire, the rate of 
deterioration from year to year. Some of the big 
wire companies have these tests made semi-annually, 
usually in the extremes of temperature, such as in 
July and January. The wires are looped at the 
distant station and simple resistance measurements 
made, such as shown in Fig. Ill, for example. 

In order to arrive at the exact resistance of each 
particular wire it is customary to measure three 
wires in loop combinations three times. For ex- 
ample, suppose we want to arrive at the exact re- 
sistance of each of three wires numbered 1, 2, and 3. 
We loop them in the following order : 
1 and 2 = 400 ohms. 

1 and 3 = 450 " 

2 and 3 = 500 " 



210 ELECTRICAL INSTRUMENTS. 

As each wire has been measured twice, we add the 
three results and halve their sum. Then if we deduct 
the result of any one of the three loop measure- 
ments from half of the sum of all three results it 
gives us the resistance of the wire which is not 
involved in the loop deducted. For example, the 
sum of the above three measurements is 1350, the 
half of which is G75. Now, if from this we deduct 
the loop measurement of 2 and 3, which was 500 
ohms, it gives us the resistance of No. 1 = 175; 
and so, if wc deduct the loop measurement of 1 and 
3, which was 450, it gives us 225 ohms as the 
resistance of No. 2, etc. 

Any one of the wires whose resistance is thus 
arrived at may then be used in a loop measure- 
ment with any other wire, whose resistance can then 
be definitely determined. 

When the resistance of each wire has been defi- 
nitely arrived at by the above method, the results 
are then divided by their length in miles and their 
resistance per mile recorded, due allowance having 
been made for all cable in circuit, etc. 

Whenever the resistance of any particular wire is 
found to be abnormally high, sectional measure- 
ments are made to determine the location of the 
added resistance, which usually consists of a bad 
joint or loose contact. This sectional measurement 
is accomplished by having successive intermediate 
stations loop the wires for measurement. 



CHAPTER XIII. 

Locating Faults in Telegraph and Telephone 
Cables. 

This subject has been pretty thoroughly covered 
in the chapter on "Cable Testing'' and there is little 
left to be said. There are a few useful points, how- 
ever, which have not been covered and which it is 
thought best to call attention to. 

Location of Grounds and Crosses by Varley Method 
Using Multiplied Arm Ratios. Telegraph and tele- 
phone cables are usually of such short lengths that 
it is necessary to make very close locations of faults 
in them in order that the measurements may be of 
much real value. This is especially true of aerial 
cables, and more especially true of aerial cables 
carrying important trunk-line telegraph wires. If 
trouble on a few of the conductors in such cables 
can be successfully located and cleared without in- 
terrupting the working circuits on the other con- 
ductors, then the test becomes of great value. 

In order to do this, a multiplied ratio must be 
used in arms A and B of bridge in the Varley test. 
The arm values are usually made 10 in A and 1000 
in B y which gives a ratio of 1 to 100. 

211 



212 ELECTRICAL INSTRUMENTS. 

In making this test the operator should be Very 
careful that he is getting perfect contacts in all of 
his bridge connections. If his bridge is of the plug 
type he should see to it that all plugs and holes are 
perfectly clean and bright. If there is any sign of 
oxidization in either, then the plugs should be 
cleaned with very fine sandpaper, such as 00, and 
a wooden peg, such as the end of a wooden pen 
staff, should be forced into each plug receptacle and 
turned around several times. If a radial-type bridge 
is used, the stubs should be bright and clean, as 
should also the under side of the radial arm which 
makes contact with them. The operator should be 



^^HtT^ 


1 — nll]ll.l. 



Fig. 120. 

absolutely sure that everything is all right before 
proceeding, and if he does this he will frequently 
save much extra trouble and annoyance. 

Suppose that we have two crossed conductors in 
a cable 1000 feet long. One of the conductors in- 
volved is looped to a good conductor at one end of 
cable, and a measurement is made from the other 



LOCATING FAULTS IN WIRES. 213 

end. A simple loop resistance measurement is first 

made, as in Fig. 120. Say that our result is 250. 

A x R 
According to the formula for this test,r = — ^ — > 

where r is the loop resistance, A = 10, R = 250, 
and B = 1000. We get 2.5 ohms for our loop. 
The connections are then made as in Fig. 121. 




Fig. 121. 

If it now takes 150 ohms in R to effect a balance, 

a. T7 o K (2.5 + 150) X 10 QO 

then F= 2.5 - 1Q00 , 10 — = •"> where F 1S 

resistance to the fault. This divided by what the 
conductor measures per foot would give the dis- 
tance to the fault. 

Let us assume in this instance that our conductor 
is a No. 11 B. & S. gauge copper wire which meas- 
ures approximately .0025 ohm per foot at normal 
temperature. Then our distance to fault would be 
.99 -f- .0025 = 396 feet. 

In making this test the operator should invari- 
ably verify his measurement by reversing the wires 



214 ELECTRICAL INSTRUMENTS. 

leading to his bridge and make a third test, seeing 
whether or not this test checks correctly with the 
previous one. Fig. 122 shows the connections for 
this "check" measurement, the formula for which is 

(L + R)XA (2.5 + 97) X 10 _ 
/<- A+B , or 100Q -.yy, where 

F represents the distance to the fault, L the loop 
measurement as in Fig. 120, R the resistance it takes 
to balance in this measurement, and A and B the 
arms of the bridge. 



l|l|l|l|l|l|H 




Fig. 122. 

It will readily be seen that instead of connecting 
the battery to the second crossed conductor as 
shown in Figs. 121 and 122, it could just as well 
be grounded and then the second crossed conductor 
grounded, as is shown in the case of overhead-wire 
measurement, Fig. 113. In cases where the con- 
ductors are grounded instead of crossed the meas- 
urements are made in the same manner as described 
above, except that in Figs. 121 and 122 the battery 



LOCATING FAULTS IN WIRES. 215 

terminal is grounded instead of being connected to 
the second crossed wire. 

How to Find Trouble after Located. It does not, 
perhaps, belong within the scope of this work to 
deal with cable faults beyond describing electrical 
methods necessary in their localization. It might, 
however, prove helpful to a great many to briefly 
describe one or two very useful and simple methods 
of finding the trouble after it has been measured 
for. This is not so important in the case of under- 
ground cables, where it is usually only necessary to 
know between which manholes the trouble lies. In 
aerial lead cable, however, it is a matter of con- 
siderable importance. 

It rarely happens that a cross between two con- 
ductors of a lead cable is a solid or metallic one. 
There is usually a medium of charred paper or 
carbonized compound which forms the cross. This 
is equally true of conductors crossed with the lead 
sheath of the cable. If, now, a current of some con- 
siderable volume, say anywhere from 100 to 250 
milliamperes, can be made to traverse the cross 
for any length of time it will be found that the lead 
sheath will quickly rise in temperature at the point 
where the trouble exists. It is, therefore, only 
necessary, after a measurement has been made, to 
ground one conductor' of a crossed pair and apply 
a grounded source of e.m.f. of 100 or more volts 
to the other crossed wire, and then have the sheath 



216 



ELECTRICAL INSTRUMENTS. 



examined for a distance of ten or fifteen feet on 
either side of the point where the measurement 
placed the fault, until the warm spot is found. 

This method has frequently been made use of to 
great advantage where the sheath of cable bore no 
sign to indicate the location of the fault. There are 
comparatively few cases of trouble in aerial lead 
cable where the method will not work out, and it 
makes the repairs possible without severing a con- 
ductor, the faulty conductors being simply sepa- 
rated, their insulation repaired, and a split sleeve 
wiped on over the bared section. 




Fig. 123. 



Another method which is referred to in some of 
the text-books (see Young's Work on Electrical 
Testing) is to send a moderately heavy intermittent 
current into the faulty conductor and then search 
for the fault by passing a coil of insulated wire in 
series with a telephone receiver along in close prox- 



LOCATING FAULTS IN WIRES. 21? 

imity to the sheath. If the current be sufficiently 
strong, and if the interruptions are of sufficient fre- 
quency, then a coil of wire of a conveniently large 
number of convolutions should pick up enough of 
the current by induction to produce a noise in the 
telephone receiver, and this noise should begin to 
diminish when the point of troubh is passed. Fig. 
123 outlines a convenient plan for applying this 
method. 

This method also presents a convenient means of 
identifying a faulty cable among a number of others 
in a common subway, or in the case of submarine 
cables in midstream when they have been brought 
up by grappling irons. 

Varley Test by Leeds & Northrup's Dial Testing 
Set, In this bridge the arms A and B, instead of 
consisting of the conventional 10, 100, and 1000 ohm 
units, are composed of a fixed set of coils so ar- 
ranged that a radial arm, representing the point of 
divide and carrying one pole of the battery, in mov- 
ing about among them creates the usual proportions. 
The bridge is also arranged so that a throw of 
certain switches places the organization in position 
for the Varley, Murray, or simple loop resistance 
measurement. 

It will be noticed in Fig. 124, which shows the 
theoretical arrangement of the bridge, that there 
is no chance for imperfect contacts in arms to give 
rise to multiplied error. The only possible imper- 



218 



ELECTRICAL INSTRUMENTS. 



feet contact is between the radial arm and stubs, and 
this would throw the added resistance in the battery 
circuit where it would do no harm. 




Fig. 124. 



The resistances in the arms are as follows : a = 
9.90.1 ohms, b = 81.008 ohms, c = 409.091 ohms, 
d = 409.091 ohms, e = 81.008 ohms, / = 9.901 
ohms. Total, 1000 ohms. It will be found by add- 
ing the resistances on each side of any given ratio 
point that the marked ratio obtains when the bat- 
tery divides at that point. 

Let us now suppose that we are measuring for a 
ground in a cable or conductor IV, which we have 
looped with a good conductor IV at the distant end 
of cable. Let us call the result of a loop resistance 
measurement with all switches to left L ; then with 
switch S still to the left, and switch S' to the 



LOCATING FAULTS IN WIRES. 



219 



right let us call the Varley measurement R, then 

_ L-DR ^ 
r — p. -. , where F is resistance to fault and D 

is the ratio dial reading. It will be noticed that 
this formula is very simple and quickly handled. 
For example, take the measurement described in 
Figs. 120, 121, and 122, where for L we got 2.5 
ohms, and for R we got 150 ohms, then with our 
ratio arm on the .01 point we would read 

~ 2.5-(. 01X150) 1 nn 1 

F= .01 + 1 -^r m = .99ohm. 

Fig. 125 shows the actual arrangement of this 
bridge. 




Fig. 125. 

Murray Test with Leeds & Northrup Dial Bridge. 

Now let us throw both switches to the right and 
place arm A on point M. If the student will trace 



220 



ELECTRICAL INSTRUMENTS. 



out the connections he will see that the arrangement 
is identical with that outlined in Fig. 91. It will 
also be found that we have 1000 ohms, or the entire 
ratio arms resistance as our fixed ratio factor. If 
now we obtain a balance on 656 ohms and call this 
Result R, then 

RxL 656X2.5 



or 



1000 + R' 1000 + 656 



= .99 ohm. 



Locating Openings in Cable Conductors by Bridge 
Method. Several methods for locating cable con- 
ductor openings have been described in different 
text-books, but the following will perhaps be found 
to be about the simplest and most reliable : 




Fig. 126. 



In Fig. 126 A and R are the arms of the bridge, 
R being made the rheostat resistance so as to get 
a wider range of adjustment than would be afforded 
by the B arm. C is a small induction coil, such as 
is used in telephones. B is an ordinary buzzer con- 



LOCATING FAULTS IN WIRES. 



221 



nected in series with the primary of C and a few 
cells of dry battery. When a balance is obtained 
there will be no noise in telephone receiver T, and 
A will then bear the same inverse ratio to R as 
L does to a, hence the formula 



F=- 



AxL 



or 



1000 X 250 



= 62.5 feet, 



R ' ~* 4000 
where F equals the distance to opening on con- 
ductor a, L the length of conductor and R the bal- 
ancing resistance. 

Another arrangement for localization of this class 
of trouble is outlined in Fig. 127. 




Fig. 127. 

In this case C is a standard condenser, against 
whose capacity we are measuring the capacity of 
the good conductor in cable. Let us call the capacity 
of C 1 M.F. Then say it takes 4000 ohms in R to 
balance the system so that there will be mini- 
mum noise in T. Then 



C'=lx 



R 



or 



1 X 1000 
4000 



-.25 M.F. 



222 ELECTRICAL INSTRUMENTS. 

This gives the capacity of the good conductor, and 

it is then only necessary to find the capacity of the 

broken conductor up to the break to ascertain how 

far out the fault lies. 

Let us say that a like measurement of the open 

conductor gives us .125 M.F. and that the cable is 

7500 feet long, then it is obvious that our fault is 

1 25 

'-—- of 7500 feet, or 3750 feet distant. 
. /c50 

In order to obviate any error which might arise 

by reason of a difference in the static capacity of 

the broken and good conductor it would be well to 

make a third test involving the good conductor 

looped to the open one at the distant end. This 

would give us the capacity of the one that is open 

beyond the fault. Let us call the result of this 

measurement .380 M.F. ; then it is obvious that 

the open conductor has a slightly greater capacity 

per foot than the good one and our formula would 

be 

AL or ./f 8 ! 780 ;^- 3676 ft., 



A + (C - B) 125 + (380 - 250) 
where F equals distance to fault, A equals capacity 
of open conductor, B equals capacity of good con- 
ductor, C equals capacity of good conductor looped 
to open conductor at distant end, and L equals length 
of cable. 

It will doubtless occur to the reader that tests B 
and C can be dispensed with if the open conductor 
is measured from both ends. 



LOCATING FAULTS IN WIRES. 223 

In order that these tests for open conductors may 
be accurate it is necessary that the opening be a 
clean break and clear of escape. 

The Leeds & Northrup Fault Finder. This instru- 
ment has been designed with a view of reducing 
calculations connected with fault location to a mini- 
mum, also so as to simplify the manipulation as 
much as possible. 

It may be used to measure conductor resistance 
to locate faults by four distinct tests and to locate 
opens using a buzzer and telephone. (With it a 
rough measurement can also be made of the resist- 
ance of faults so that the operator can decide how 
closely he should be able to make the location.) 

Resistance Measurement with the Leeds and 
Northrup Fault Finder. 

The essential feature of the apparatus is the uni- 
form resistance A B, which is wound in a circle and 
is about 100 ohms. By a special construction it is 
arranged so that contact can be made at any point 
along it, and it is, therefore, equivalent to a high 
resistance wire. It has a moving contact C and 
a scale of 1000 divisions. In series with this there 
are the two resistances E and R. E has exactly the 
same resistance as the wire A B. R has a resistance 
of 100 ohms. Either resistance may be short-cir- 
cuited by a small switch. The resistances show T n 



224 



ELECTRICAL INSTRUMENTS. 



between the ground post and the battery and be- 
tween post BA and the battery key are simply to 
protect the battery and the apparatus from excessive 
current. Fig. 128 shows the proper connections for 
measuring conductor resistance. As in the ordi- 
nary slide wire bridge, the resistance X between the 




Fig. 128. 

two posts i and 2 is gotten from the formula 

A 

-j R. To avoid the necessity of solving 

1000 — /i 

A 



X 



in each case the fraction 



:, a table is fur- 



1000 — -4' 

nished with the set, giving the value of this fraction 



LOCATING FAULTS IN WIRES. 



225 



for each value of A. The resistance is accordingly 
determined in each case by simply setting the con- 
tact C for a balance and reading from the table the 
value opposite the number on the scale and multi- 
plying by 100. 

Fault Location with the Leeds & Northrup Fault 
Finder. 

FIRST METHOD. 




Fig. 129. 



This method is to be used in locating faults where 
there are two wires having equal resistance, in one 
of which there is a fault. Connect and set switches 
as shown in Fig. 129. It will be remembered that 
E is equal to the resistance of the wire A B. From 
the symmetry of the arrangement it will be obvious 
that the contact point C would rest for a balance at 
1000 on the scale if the fault were exactly at the June- 



226 



ELECTRICAL INSTRUMENTS. 



tion between the good and the bad wires ; it would rest 
at 500 if the fault were half way along the bad wire ; 
and at whatever point it comes to rest, the reading, 
divided by 1000 and multiplied by the length of the 
bad wire, is the distance from the instrument to the 
fault. 

SECOND METHOD. 




Fig. 130. 



This method is to be used for locating faults 
where the good and the bad wires are not equal to 
each other. The connections are shown in Fig. 
130. It is the ordinary Murray loop, and it will 
readily be seen that the resistance a to the fault will 

be gotten from the formula a = -— Z, where L is 

the resistance of the loop and A is the reading of the 
contact C on its scale. 



LOCATING FAULTS IN WIRES. 



227 



THIRD METHOD. 

This method may be used as a check on either of 
the above. The connections are shown in Fig. 131. 




Fig. 131. 

The resistance a to the fault from the binding-post 

B.L — 100A t 
1 is a = , where L equals the total re- 
sistance of the loop, A equals the reading of contact 
C, and£=: 1000 — A. 




Fig. 132. 
To Locate Opens, Using Buzzer and Telephone, 



228 



ELECTRICAL INSTRUMENTS. 



In this case the slide resistance circle A B is used 
without either of the resistances E or R. Connect 
to binding-post 1 the broken wire and to binding- 
post 2 a perfect wire having the same capacity per 
mile. Connect buzzer as shown and telephone X 
between the binding-posts 1 and 2. For a mini- 




Fig. 133. 

mum sound in the telephone we will have the 

A a 
relation -^ — j, where a is the length of the broken 

wire and L that of the whole cable. From this, 
a = __ . L. As in the case of resistances, the 



LOCATING FAULTS IN WIRES. 229 



value can be read from the table. The 

1000 - A 

connections are shown in Fig. 132. 

Fig. 133 shows the actual arrangement of the 

Leeds & Northrup Fault Finder. 



INDEX. 



Abscissae 188 

Allowances in cable tests 148 

Ammeter 39 

checking 126 

connections to equalizer 74 

series 39 

shunt 40, 58 

Thomson 64 

voltmeter used as 185 

Ampere turns 3 

Armature coils, resistance of 182 

insulation of 166 

Arms of bridge 81 

" setting of 81, 91 

Armor connection to ground 143 

Astatic ammeter 67 

" galvanometer 9 

Automatic signaling device 196 

Averaging cable tests 144, 147 

Ballistic galvanometer 17 

Battery, negative to cable 144 

" reversed in cable test 147 

" standard 38 

" tests of 116 

Bridge, Wheatstone 75 

Leeds & Northrup 217 

Post Office 78 



2%1 INDEX. 

Bridge, Wheatstone, Queen 88 

slide wire 95 

Stearns 96 

Whitney 91 

Willyoung 85 

Cable, capacity tests 153 

" battery in testing 144 

" electrification of 144 

" faults, the locating of 154, 211, 228 

" multiple, testing of 144 

testing 142 

" allowances in 148 

" operations 148 

Capacity, measurement of 138 

Cardew voltmeter 53 

Care of instruments 73 

Calculating shunts 28 

Cell, Clark 38 

" checking e.m.f. of 125 

" measuring resistance, etc., of 117 

" standard 38 

" Weston 38 

Charge, loss of 140 

Chart, plotting.. 187 

Charging condenser 135 

Checking ammeter 126 

e.m.f 125 

voltmeter 126, 129 

Clark cell 38 

Coil, suspension of 12, 20 

" armature, testing 166, 182 

" field, testing 179 

Coils, resistance 24 

Commutator 35 

Compensating magnet 10 



INDEX. 233 

Condenser 29, 136 

adjustable 31 

charge and discharge 135 

comparison of 139 

Conductivity test 209 

Constant 12, 106 

deflection 109 

Copper wire resistance 5 

Cords for shunt 59 

Corrections for drop method 187 

Current, measurement by voltmeter 185 

Curves, plotting of 187 

D'Arsonval galvanometer 16 

Deflection 2 

constant 109 

direct 105 

Deflections not uniform 2, 6 

Differential voltmeter 50 

Dielectrics 29, 141 

Direct deflection constant 12, 106 

method 105 

Discharge key 36, 136 

of condenser 136 

Drop across lamp 174 

" along line 185 

Duplex instruments 48 

Earth or ground connections 149 

" currents 147 

" influence of 9 

" magnetic constant of 121 

Edgewise voltmeter 72 

Electrification of cable 144 

Electro-dynamometer system 68 

Electro-magnetic system 70 



234 



INDEX. 



Electrostatic system 70 

E.m.f. around armature 173 

" in circuit 102 

" testing high with low reading voltmeter 175 

Equalizer, ammeter connections 74 

Evershed ohmmeter 99 

Faults, location of 154, 193. 228 

Figure of merit 12, 106 

Field coils, testing 179 

Fault finder, Leeds & Northrup 223 



Galvanometer, astatic 9 

ballistic 17 

constant 12 

D'Arsonval 10 

moving coil 13 

resistance of 3, 13, 114 

scale 22 

sensibility 12 

shunts 27, 107 

tangent 0, 121 

Thomson reflecting 12 

wall pattern 19 

windings for 4 

Generator, e.m.f. around armature 173 

insulation of 165 

German silver wire 25 

Ground connection 149 



Hot wire voltmeter 53 



Incandescent lamp test 174 

Inclined coil ammeter 64 

" voltmeter 65 

Inductive capacity 141 



INDEX. 235 

Insulation and length of cable 144 

and temperature 144 

by loss of charge 140, 152 

of cable averaged 147 

of armature 166 

of generator 165 

of line - 166 

resistance of cable 142 

test with telephone 145 

" with milliammeter 207 

" voltmeter 208 

Kelvin galvanometer 

(See Thomson galvanometer.) 

Key, discharge 36 

Kempe 36, 136 

reversing 32 

rubbing contact 35 

Rymer-Jones 34 

short circuit 32 

Keystone voltmeter 69 

Lamp, drop across 174 

efficiency of incandescent 174 

Line insulation, testing of 166 

Loss of charge, insulation by 140, 152 

Location of faults 154 

Loop tests 154 

Loss or drop of e.m.f 174, 185 

Magnet, compensating 10 

Magnetic vane instrument 61 

Magnetic influence of .earth 9, 121 

Manganin 25 

Measuring high e.m.f 1"5 

Megohms, calculating in 109 



236 INDEX. 

Merit, figure of 12 

Miles, reducing results to 144, 201, 205 

Milliampere, and meters 52 

Millivolt, and meters 52 

Multiple cable testing 144 

Multiplier for voltmeter 51 

Multiplying power of shunt 29 

Murray fault test 158, 219 

Negative pole of battery to cable 144 

Ohm's Law, applications of 3, 103 

Ohmmeter, Evershed 99 

Sage 97 

Ordinate 188 

Oscillatory discharge of condenser 136 

Paralax 71 

Platinoid 25 

Platinum silver 25 

Plotting curves 187 

Plugs for resistance coils 24 

Polarized instruments 74 

Portable sets — see Wheatstone Bridge. 

Positive pole of battery to ground 144 

Potentiometer 123 

Potential indicator 49, G3 

Power of shunt, multiplying 29 

Proportional arms of bridge 81 

Queen voltmeter 70 

" portable testing set 88 

Reading instruments % 71 

Reducing result to mile iengths 144, 201, 205 



INDEX. 237 

Resistance and temperature 176 

of battery 117 

of galvanometer 3, 13, 114 

of copper wire 5 

of voltmeter 45 

slab 26 

testing of 104, 113 

wires 24 

Reversing battery in cable test 144 

key 34 

Rheostat 3, 23 

Rymer-Jones key 34 

Sage ohmmeter 97 

Scale, galvanometer 22 

Sensibility of galvanometer 12 

Series ammeter 39 

Shunts, ammeter 58 

care of 59 

calculating 28 

galvanometer 27 

multiplying power 29 

use of 27, 107 

Solenoidal voltmeter 61 

Standard cell 38 

Stearns bridge 96 

Storage battery, instruments for 74 

Suspension of coil 12, 20 

Tangents 6 

Tangent galvanometer 6, 12i 

Telephone, testing with 145, 215 

Temperature and resistance 176 

" insulation 144 

Testing Armature 166-172, 182 

by direct deflection 105 



238 INDEX. 

Testing by loss of charge 140, 152 

cable 142,148,153,211,229 

" " summary of 148 

capacity 138 

" current in lamp 174 

" drop across lamp 174 

" earth currents 149 

field coils 179 

for crosses 195, 199, 202, 204 

11 grounds 194, 198, 225 

" opens 194, 220, 227 

" lengths of wire 96 

" wiring 166 

with bridge 79, 85, 88, 91 

" " galvanometer 102 

" milliammeter 207 

" ohmmeter 97-99 

" potentiometer 129 

" voltmeter 103, 205, 208 

Thermometer test 178 

electric 179 

Thomson reflecting galvanometer 12 

inclined coil ammeter 64 

Turn, ampere 3 

Unreeling, test of cable by 160 

Varley fault test 157, 211, 217 

Voltmeter, Cardew 53 

checking 120, 129 

differential 50 

hot wire 53 

Keystone 69 

mutiplier for 51 

principle of 39 

Queen 70 



INDEX. 239 

Voltmeter, low reading and high e. m. f 175 

resistance of 39-45-172 

sensibility of 45 

Thomson 64 

used as ammeter 185 

Westinghouse 02 

Weston 41 

Whitney 54 

Wattmeter 68 

Westinghouse voltmeter 62 

Weston standard cell 38 

Wheatstone Bridge 75 

" Leeds & Northrup 217, 220 

" P. O. form 78 

" Queen 88 

" slide wire 95 

" Stearns 96 

" Whitney '. . . 91 

" Willyoung 85 

Wire, resistance of copper 5 

Wires, resistance. 24 

Wire, testing in lengths 96 

Wiring, testing of 166 

Zero, central 49, 74 

" method 125 

" resistance of fault 154 




WESTON 

Electrical Measuring Instruments 

The continued development 
and improvement in the well- 
known Weston Instruments has 
resulted in the present prac- 
tically perfect models. 

WESTON STANDARD PORTABLE VOLTMETERS AND 
AMMETERS are the best instruments available for use in testing 
circuits. 

WESTON LABORATORY 
STANDARD INSTRUMENTS 
are the most sensitive and ac- 
curate obtainable. They are 
recognized and used as Stand- 
ards throughout the world. 

LOW PRICED PORTABLE 
INSTRUMENTS are sold by 
the Weston Company for use 
where extreme accuracy is not 
required. Even in these low 
priced instruments the usual per- 
fection ot workmanship peculiar to Weston products is exhibited. 

Weston Switchboard Instruments of the following types : ILLUMINATED 
DIAL, ROUND PATTERN, EDGEWISE, DUPLEX, in lar.e variety of sizes 
and ranges are the best made mechanically, as well as electrically. They are 
unsurpassed in points of low consumption of energy and indicate instantaneously 
any variation in the circuits. 

Catalogues giving full description of all types of Instruments 
will be mailed promptly upon application to 

Weston Electrical Instrument Company 

MAIN OFFICE AND WORKS, 

Waverly Park, NEWARK, N. J., U. S. A. 

NEW YORK OFFICE, 74 CORTLANDT ST. 




THE 

LEEDS & NORTHRUP 

CO. 



Electrical Measuring 
Instruments 

INCLUDING 

Portable Testing Sets and 
Cable Testing Apparatus 




L. & N. DECADE PORTABLE TESTING SET 

Potentiometers Galvanometers 

Standard Condensers Self-Induction Apparatus 
Standard Resistances Resistance Thermometers 
Conductivity Bridges Dynamometers 
Photometric Apparatus 

4901 STENTON AVE., PHILADELPHIA 



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94 pages, 42 illustrations, 12mo., cloth, $1.00. 

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ELECTRICAL INSTRUMENTS— 
and TESTING. 

How to Use the Voltmeter, Ammeter, Galvanometer, Potentiometer, 
Ohmmeter, the Wheatstone Bridge, and the Standard Portable 
Testing Sets. 

BY 

NORMAN H. SCHNEIDER. 
Author of " Care and Handling of Electric Plants," u Induction 
Coils and Coil Making," M Circuits and Diagrams," etc., etc. 



The aim of the author has been to produce a complete and prac- 
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Using only formulas in simple algebra and then explaining them 
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During the past ten years the author has made hundreds of 
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practical standpoint. He has also obtained valuable information 
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Most of the diagrams have been specially drawn for this book. 

The work is divided into XI. chapters as follows: 

Introduction; Chapters I. and II, The Galvanometer; III, 
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CONTENTS. 
Alarms. — Doors and Windows ; Cisterns ; Low Water in Boilers ; Time 
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lan's ; Copper-oxide; Cruikshank's ; Daniel's; Granule carbon; Groves; 
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electric. Bells — Annunciator System ; Double System ; and Telephone; 
Making ; Magnet for ; Bobbins or Coils ; Trembling ; Single Stroke ; 
Continuous Ringing. Connections. Carbons. Coils. — Induction ; Prim- 
ary ; .Secondary ; Contact-breakers ; Resistance. Intensity Coils. — Reel; 
Primary ; Secondary ; Core ; Contact-breaker ; Condenser ; Pedestal ; 
Commutator ; Connections. Dynamo- Electric Machines — Relation of 
Speed to Power ; Field-Magnets ; Pole-pieces ; Field-magnet Coils ; 
Armature Cores and Coils ; Commutator Collectors and Brushes ; Relation 
of size to efficiency ; Methods of exciting Field-Magnets; Magneto-Dyna- 
mos ; Separately excited Dynamos ; Shunt Dynamos ; Organs of Dyna- 
mos as constructed in practice ; Field-Magnets ; Armatures ; Collectors ; 
Brush Dynamo ; Second Class ; Alternate Currents ; Third Class., Fire 
Risks. — The Dynamo ; Wires ; Lamps ; Danger to persons. Measuring. 
— Non-Registering Instruments ; Registering Instruments. Microphones. 
— Construction, &c. Motors. — Application ; for Railways. Phonographs 
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135 PAGES. 126 ILLUSTRATIONS. 8 VO. 

Cloth, 75 Cents. 



AN AMERICAN BOOK. 



I2SS32 OF Md pint 

Second edition thoroughly revised, greatly enlarged and brought u* 
to latest American Practice, 



By H. S. NORRIE, 

(NORMAN H SCHNEIDER) 



Considerable space in the new matter is given to the following . 
Medical and bath coils, gas engine and spark ceils, contact breakers, 
primary and secondary batteries; electric gas lighting; new method 
of X-ray work, etc. A complete chapter on up-to-date wireless tele- 
graphy; a number of new tables and 25 original illustrations. Great 
care has been given to the revision to make this book the best Amer- 
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illustrations and contents of tables have been added. 

Contents of Chapters. 

1. Construction of coils; sizes of wire; winding; testing; insula- 
tion; general remarks; medical and spark coils. 2. Contact breakers. 
3. Insulation and cements. 4. Construction of condensers. 5. Ex- 
periments. 6. Spectrum analysis. 7. Currents in vacuo; air pumps. 
8. Rotating effects. 9. Electric gas lighting; in multiple; in series. 
10. Primary batteries for coils; varieties; open circuit celh; closed 
circuit cells; solutions. 11. Storage or secondary batteries; construc- 
tion; setting up; charging. 12. Tesla and Hertz effects. ij. Roent- 
gen Radiography. 14. Wireless telegraphy; arrangement of circuits 
of coil and coherer for sending and receiving messages; coherers ; 
translating devices; air conductors; tables; contents; index. 

XII + 270 Pages, 79 Illustrations, 5x6^ Inches. 
Cloth. $1.60. 



PRINCIPLES OF 

ELECTRICAL POWER. 

(CONTINUOUS CURRENT.) 

FOR MECHANICAL ENGINEERS. 

BY 

A. H. BATE, A.M.I. E.E. 



The rapid progress that has been made of late years in the appli- 
cation of electricity to industrial purposes, and particularly in the 
transmission of power by means of the electric motor, has made it 
imperative for every engineer who wishes to keep up to date to 
have some knowledge of the way electrical currents are controlled 
and used for practical purposes. This work is especially written 
for the practical engineer, mat hematics being avoided. 
Contents of Chapters. 

1. The Electric Motor. 

2. Magnetic Principles. 

3. Electrical Measurements. 

4. The Dynamo. 

5. Construction of Motor. 

6. Governing of Motors. 

7. Open and Closed Motors; rating. 

8. Motor Starting Switches. 

9. Speed Control of Shunt-wound Motors. 

10. Series Motor Control. 

11. Distribution System. 

12. Installing and Connections. 

13. Care of Dynamos and Motors. 

14. Cost of Plant. 

15. Examples of Electric Driving. 

Horse-power absorbed by various machines, including general 
engineering and shipyard machines; wood working and printing 
machinery (arranged in 14 pages of tables). 

XII + 204 pages, 63 illustrations, 12 mo. cloth. $2.00.* 



THE PRACTICAL ENGINEER'S HANDBOOK. 
TO THE CARE AND MANAGEMENT OF 

ELECTRIC P OWER P LANTS 

By NORMAN H. SCHNEIDER, 

Chief Engineer, "White City," Coling*voood, Ohio* 

EXTRACTS FROM PREFACE. 

In revising the first edition of Power Plants the author decided 
to greatly enlarge it in the hope that it will have a still greater 
success than the first one. The section on theory is thoroughly 
revised. A complete chapter on Standard Wiring including new 
tables and original diagrams added. The National Fire Under- 
writers' rules condensed and simple explanations given. 

Direct and alternating current motors have been given a special 
chapter and modern forms of starting rheostats described at length. 
The principles of alternators have been considered also trans- 
formers and their applications. Modern testing instruments and 
their use are given a separate chapter. New matter has been 
added to storage batteries including charging of automobile bat- 
teries, 10 new tables, and 137 new illustrations. 

SYNOPSIS OF CONTENTS OF CHAPTERS. 

1. The Electric Current; series and multiple connections; 
resistance of circuits; general explanation of formulas. 

2. Standard Wiring; wiring formulas and tables; wiring sys- 
tems; cut-outs; conduits; panel boxes; correct methods of wiring. 

3. Direct and Alternating Current Generators; manage- 
ment in the power house; windings; selection of generators. 

4. Motors and Motor Starters; various forms of motors; con- 
trollers; care of motors and their diseases; rules for installing. 

5. Testing and Measuring Instruments; voltmeter testing 
and connections; instruments used; switchboard instruments. 

6. The Storage Battery; different kinds; switchboards for 
charging fixed and movable batteries; management of battery. 

7. The Incandescent Lamp; various methods of testing; life 
of lamps. 

8. Engineering Notes; belts and pulleys h.p. of belts. Tables. 
Contents. Index. 

290 pages, 203 illustrations. 12mo., cloth, $1.50. 
Full limp leather, $2.50. 



Design of Dynamos 

BY 

SILVANUS P. THOMPSON, D. Sc-, B. A., F. R. S. 

EXTRACTS FROM PREFACE. 

" The present work is purposely confined to continuous current 
generators. The calculations and data being expressed in inch 
measures; but the author has adopted throughout the decimal sub- 
division of the inch; small lengths being in mils, and small areas of 
cross-section in sq. mils, or, sometimes, also, in circular mils." 

CONTENTS OF CHAPTERS. 

1. Dynamo Design as an Art. 

2. Magnetic Data and Calculations. Causes of waste of 
Power. Coefficients of Dispersion. Calculation of Dispersion. 
Determination of exciting ampere-turns. Example of Calculation, 

3. Copper Calculations. Weight of Copper Wire. Electrical 
resistance of Copper, in cube, strip, rods, etc. Space-factors. I 
Windings; Ends; Insulation; Ventilating; Heating. 

4. Insulating Materials and Their Properties. A list of 
materials, including " Armalac," " Vitrite," " Petrifite," " Mican- 
ite," " Vulcabeston," " Stabilite," " Megohmite," etc. With tables. 

5. Armature Winding Schemes. Lap Windings, Ring Wind- 
ings, Wave Windings, Scries Ring- Windings, Winding Fommlae. 
Number of circuits. Equalizing connections. Colored plates. 

6. Estimation of Losses, Heating and Pressure-drop. Cop- 
per Losses, Iron Losses, Excitation Losses, Commutator Losses, 
Losses through sparking. Friction and Windage Losses. Second- 
ary Copper Losses. 

7. The Design of Continuous Current Dynamos. Working 
Constants and Trial Values; Flux-densities; Length of Air-gap; 
Number of Poles; Current Densities; Number of Armature Con- 
ductors; Number of Commutator Segments; Size of Armature 
(Steinmetz . coefficient) ; Assignment of Losses of Energy, Cen- 
trifugal Forces; Calculation of Binding Wires; Other procedure in 
design. Criteria of a good design. Specific utilization of material. 

8. Examples of Dynamo Design. 

1. Shunt-wound multipolar machine, with slotted drum arma- 
ture. 2. Over-compounded Multipolar traction generator, with 
slotted drum armature, with general specifications, tables, dimen- 
sions and drawings, fully described. 

A number of examples of generators are given in each chapter, 
fully worked out with rules, tables and data. 

VIII. X253 pages, 92 illustrations, 10 large foiling plates and 4 
Three-color Plates, 8vo., cloth, $3.50. 



Dynamo=Electric Machinery 

VOL. L— CONTINUOUS CURRENT. 



SILVANUS P. THOMPSON, D.Sc.,B.A.,F.R.S, 



7th Edition Revised and Greatly Enlarged* 



CONTENTS OF CHAPTERS. 

I. Introductory. 2. Historical Notes. 

3. Physical Theory of Dynamo-Electric Machines. 

4. Magnetic Principles; and the Magnetic Properties of Iron. 

5. Forms of Field-Magnets. 

6. Magnetic Calculations as Applied to Dynamo Machines. 

7. Copper Calculations; Coil Windings. 

8. Insulating Materials and their Properties. 

9. Actions and Reactions in the Armature. 

10. Commutation; Conditions of Suppression of Sparking. 

II. Elementary Theory of the Dynamo, Magneto and Separately 
Excited Machines, Self-exciting Machines. 

12. Characteristic Curves. 

13. The Theory of Armature Winding. 

14. Armature Construction. 

15. Mechanical Points in Design and Construction. 

16. Commutators, Brushes and Brush-Holders. 

17. Losses, Heating and Pressure-Drop. 

18. The Design of Continuous Current Dynamos. 

19. Analysis of Dynamo Design. 

20. Examples of Modern Dynamos (Lighting and Traction). 

21. Dynamos for Electro-Metallurgy and Electro-Plating. 

22. Arc-Lighting Dynamos and Rectifiers. 

23. Special Types of Dynamos; Extra High Voltage Machines, 
Steam-Turbine Machines, Extra Low Speed Machines, Exciters, 
Double-Current Machines, Three-Wire Machines, Homopolar (Uni- 
polar) Machines, Disk Dynamos. 

24. Motor-Generators and Boosters. 

25. Continuous-Current Motors. 

26. Regulators, Rheostats, Controllers and Starter. 

27. Management and Testing of Dynamos. 

Appendix, Wire Gauge Tables. Index. 

996 pages, 573 illustrations, 4 colored plates, 32 large folding 
plates. 8vo., cloth. $7.50.1 



Alternating - Current Machinery 

BEING VOL. II OF 

Dynamo-Electric Machinery* 

BY 

SILVANUS P. THOMPSON, OSc, BJL, F.R.S. 



Owing to the enormous increase in the use of electrical machinery 
since the publication of the sixth edition of Dynamo-Electric 
Machinery the author has deemed it advisable to divide the work. 
Vol. I. is devoted to Direct Current Machinery and this the 
second part. Vol. II. Alternating Current Machinery. 
Amongst the many new features treated special mention must be 
made of the number of fine colored plates of windings and the many 
large folding scale drawings. These two volumes make the most 
comprehensive and authoritative work on dynamo machinery. 
The work has been so universally adopted that it has been found 
necessary to translate it into French and German. 

CONTENTS OF CHAPTERS. 

1. Principles of Alternating Currents. 

2. Periodic Functions. 

3. Alternators. 

4. Induced E.M.F. and Wave-Forms of Alternators. 

5. Magnetic Leakage and Armature Reaction. 

6. Winding Schemes for Alternators. 

7. Design of Alternators. Compounding of Alternators. 

8. Examples of Modern Alternators. 

9. Steam Turbine Alternators. 

10. Synchronous Motors, Motor Generators, Converters. 

11. Parallel Running of Alternators. 

12. Transformers. 13. Design of Transformers. 

14. Induction Motors. 15. Design of Induction Motors. 

16. Examples of Induction Motors. 

17. Single-Phase Induction Motors. 

18. Alternating-Current Commutator Motors. 

Appendix. The Standardization of Voltages and Frequencies. 

Complete Index. 
XX + 848 pages, 546 illustrations, 15 colored plates and 24 large 
folding plates. 8vo. t cloth. $7.50{. 



Books for Steam Engineers. 



DIGRAM OF CORLISS ENGINE. A large engraving giving 
a longitudinal section of the Corliss engine cylinder, showing rela- 
tive positions of the piston, steam valves, exhaust valves, and 
wrist plates when cut-off takes place at % stroke for each 15 degrees 
of the circle. With full particulars. Reach-rods and rock shafts. 
The circle explained. Wrist-plates and eccentrics. Explanation of 
figures, etc. Printed on heavy paper, size 13 in. x 19 in., 25c. 

THE CORLISS ENGINE and its Management. A Practical 
Handbook for young engineers and firemen, (3rd edition) by J. T. 
Henthorn. A good little book, containing much useful and practi- 
cal information. Illustrated, cloth, $1 .00. 

THE FIREMAN'S GUIDE to the Care and Management of 
Boilers, by Karl P. Dahlstkom, M.E., covering the following sub- 
jects: Firing and Economy of Futl; Feed and Water Line: Low 
Water and Priming: Steam Pressure: Cleaning and Blowing Out; 
General Directions. A thoroughly practical book. Cloth, 50c. 

A B C OF THE STEAM ENGINE. With a description of the 
automatic shaft governor, with six large scale drawings. A prac- 
tical handbook for firemen helpers and young engineers, giving a 
set of detail drawings all numbered and lettered and with names 
and particulars of all parts of an up-to-date American high speed 
stationary steam engine. Also a large drawing and full descrip- 
tion of the automatic shaft governor. With notes and practical 
hints. This work will prove of great help to all young men who 
wish to obtain their engineer's license. Cloth, price 50c. 

HOW TO RUN ENGINES AND BOILERS. By E. P. Watson, 

(for many years a practical engineer, and a well-known writer in The 
Engineer.) A first-rate book for beginners, firemen and helpers. 
Commencing from the beginning, showing how to thoroughly overhaul 
a plant, foundations, lining up machinery, setting valves, vacuum, 
eccentrics, connection, bearings, fittings, cleaning boilers, water tube 
boilers, running a plant, and many useful rules, hints and other 
practical information; many thousands already sold. 160 pages, 
fully illustrated, cloth, $1.00. 

AMMONIA REFRIGERATION. By I. I. Redwood. A practi- 
cal work of reference for engineers and others employed in the man- 
agement of ice and refrigerating machinery. A first-rate book, be- 
ginning from the bottom and going carefully through the various 
processes, stage by stage, with many tables and original illustrations. 
Cloth, $1.00. 

MEYER SLIDE VALVE. Position diagram of cylinder with 
cutoff at 1^, J^, % and ^ stroke of piston with movable valves, on 
card 7K in * x S% in. Pric«, 25c, 



AN ELEMENTARY TEXT-BOOK 

ON 

STEAM ENGINES AND BOILERS 

FOR THK 

USE OF STUDENTS IN SCHOOLS AND COLLEGES. 

BY 

J. H. KINEALY. 

Professor of Mechanical Engineering, Washington University. 



Illustrated with Diagrams and Numerous Cuts, Showing American Types 
and Details of Engines and Boilers. 



This book is written solely as an elementary text book for the use of be- 
ginners and students in engineering, but more specially for the students in 
the various universities and colleges in this country. 

No attempt has been made to tell everything about any one particular 
subject, but the author has endeavored to give the student an idea of 
elementary thermodynamics, of the action ot the steam in the cylinder of 
the engine, of the motion of the steam valve, of the differences between the 
various types of engines and boilers, of the generation of heat by combus- 
tion, and the conversion of water into steam. 

Care has been taken not to touch upon the design and proportion of the 
various parts of engines and boilers for strength ; as, in the opinion of the 
writer, that should come after a general knowledge of the engine and 
boiler has been obtained. 

In the derivation of some of the formaUe in thermodynamics, it has been 
necessary to use the calculus, but the u-e of all mathematics higher than 
algebra and geometry has been avoided as much as possible. 

An earnest endeavor has been made to present the subject in a clear 
and concise manner, using as few words as possible and avoiding all 
padding. 

Contents of Chapters. 

Chapter I. — Thermodynamics ; First Law of Thermodynamics ; Work, 
Power ; Unit of Heat ; Mechanical Equivalent ; Application of Heat to 
Bodies ; Second Law of Thermodynamics ; Specific Heat ; Absolute Tem- 
perature ; Application of Heat to a Perfect Gas ; Isothermal Expansion ; 
Adiabatic Expansion ; Fusion ; Vaporisation ;. Application of Heat to 
Water; Superheated Steam. Chapter II. — Theoretical Heat Engine; 
Cycle ; Thermodynamic Efficiency ; Perfect Gas Engine ; Perfect Steam 
Engine ; Theoretical Diagram of the Real Engine ; Clearance ; Efficiency 



The Slide Valve 

SIMPLY EXPLAINED. 

By W.J. TENNANT, Asso. M. Inst. Mech. E. 

The work has been thoroughly revised and enlarged 
in accordance with the present American Practice. 

By J. H. KINEALY,D. E., M. Am. Soc. Mech. E. 



The work is based upon notes and diagrams which were prepared 
by Mr. Tennant in his lectures to his classes of working engineers 
and students towards the obtainment of clear general notions upon 
the Slide Valve, its design, varieties, adjustments and management. 
They have been revised and considerably added to and in this form 
the authors believe they will be of considerable value to all 
engineers and others interested in steam engines. 



CONTENTS OF CHAPTERS* 

I. The Simple Slide. 

II. The Eccentric a Crank. Special Model to give Quantitative 

Results. 

III. Advance of the Eccentric. 

IV. Dead Centre. Order of Cranks. Cushioning and Lead. 

V. Expansion— Inside and Outside Lap and Lead; Advance 

affected thereby. Compression. 

VI. Double-ported and Piston Valves. 

VII. The Effect of Alterations to Valve and Eccentric. 

VIII. Note on Link Motions. 

IX. Note on very early cut-off, and on Reversing Gears in 

general. 

The illustrations aim to cover the different kinds of Slide Valves* 
and the circular diagrams will prove a novel feature. 

88 Pages. 41 Illustrations. 12mo. Cloth, $1.00 



LUBRICANTS, 
OILS AND GREASES. 

TREATED THEORETICALLY AND GIVING PRACTICAL 
INFORMATION REGARDING THEIR 

COMPOSITION, USES AND MANUFACTURE. 

A PRACTICAL GUIDE FOR MANUFACTURERS, ENGINEERS, 
AND USERS IN GENERAL OF LUBRICANTS. 

By ILTYD I. REDWOOD, 

Associate Member American Society of Mechanical Engineers; Member Society Chemical 

Industries (England); Author of 'Theoretical and Practical Ammonia Refrigeration/ 

and a 'Practical Treatise on Mineral Oils and Their By-Prodrcts. 



CONTENTS. 

Introduction. — Lubricants. 

THEORETICAL. 
Chapter I. — Mineral Oils : American and Russian ; Hydrocarbons. 
Chapter II. — Fatty Oils : Glycerides ; Vegetable Oils ; Fish Oils. 
Chapter III. — Mineral Lubricants : Graphite ; Plumbago. 
Chapter IV. — Greases : Compounded ; M Set " or Axle ; " Boiled " 
or Cup. 
Chapter V.— Tests of Oils: Mineral Oils. Tests of Oils: Fatty Oils 

MANUFACTURE. 

Chapter VI. — Mineral Oil Lubricants: Compounded Oils; De- 
bloomed Oils. 

Chapter VII.— Greases : Compounded Greases; "Set" or Axle 
Greases ; Boiled Greases ; Engine Greases. 

Appendix. — The Action of Oils on Various Metals. Index. 

Tables : I. — Viscosity and Specific Gravity. II.— Atomic Weights. 
III.— Origin, Tests, Etc of Oils. IV.— Action of Oils on Metals. 

List of Plates : I — I. I. Redwood's Improved Set Measuring 
Apparatus II.— Section Grease Kettle. III.— Diagram of Action 
of Oils on Metals. 

8vo. Cloth. $1.50. 



Mechanical Draft. 

BY 

J. H. KINEALY, M. Am. Soc. M.E. 

Past President American Society Heating and Ventilating Engineers.. 



PREFACE. 
In writing this book the author has assumed that those who 
will use it are familiar with boilers and engine plants, and he 
has had in mind the practicing engineer who is called upon to design 
power plants, and who must therefore decide when it is best to use 
some form of mechanical draft. The arrangement of the book is 
what the experience of the author in making calculations for mech- 
anical draft installations has shown him is probably the best. 
And he has tried to arrange the tables in such a way and in such 
a sequence that they may prove as useful to others as they have 
to him. 

CONTENTS OF CHAPTERS. 

1. General Discussion. Introduction; systems of mechanical 
draft; chimneys v. mechanical draft; mechanical draft and econ- 
omizers. 

2. Forced Draft. Systems; closed fire-room system; closed 
ashpit system; small fan required; usual pressure; forced draft and 
economisers; advantages; disadvantages. 

3. Induced Draft. Introduction; temperature of gases; advan- 
tages; disadvantages. 

4. Fuel and Air. Weight of coal to be burned; evaporation 
per lb. of coal; effect of rate of evaporation; weight of air required; 
volume of air and gases ; volume of gases to handle ; leakage ; factor 
of safety. 

5. Draft. Relation to rate of combustion; resistance of grate; 
resistance due to economizer; draft required under different con- 
ditions. 

6. Economizers. Effect of adding; ordinary proportion and 
cost; increase of temperature of feed water. 

7. Fans. Type and proportions of fan used; relation between 
j revolution of fan and draft; capacity of fan. 

8. Proportioning the Parts. Diameter of fan wheel required; 
speed at which the fan must run; power required to run the fan; 
size of engine required ; steam used by fan engine ; choosing the fan 
for forced draft, for induced draft without economizer, for induced 
draft with economizer; location of the fan; breeching and up-take; 
Unlet chamber; discharge chimney; by-pass; water for bearings. 

Appendix. Tables. Index. 156 pages. 13 plates. 16mo. 
Cloth, $2.00. 



THE AUTHORITY ON THIS SUBJEGT. 



CENTRIFUGAL FANS. 

A THEORETICAL AND PRACTICAL TREATISE ON 

Fans for Moving Air In Large Quantities 
At Comparatively Low Pressures. 

BY 

J. H. KINEALY, M. Am. Soc. M.E. 

Past-President American Society Heating and Ventilating Engineers. 

The matter in this book was a series of articles written for the 
Engineering Review. The favorable attention which they at- 
tracted lead the author to believe that there was a real demand 
for a book treating in a theoretical as well as a practical way on 
centrifugal fans. The articles have been thoroughly revised, added 
to, and made as complete as possible. 

Contents of Chapters. 

1. Flow of Air; Volume of Air Flowing; Pressure Necessary for 
required velocity. 

2. Vortex; Vortex with Radial Flow. 

3. Fans; First Type of Fans; Second or Guibal Type of Fans; 
Third Type of Fans; Modern Type. 

4. Fan Wheel; Vanes or Floats; Inlet; Width. 

5. Capacity; Blast Area; Effect of Outlet on Capacity; Air 
per Revolution. 

6. Pressure; Work. 

7. Horse Power Required to Run a Fan ; Engine Required 
to Run a Fan; Motor Required to Run a Fan; Width of Belt. 

8. Efficiency; Air per Horse Power. 

9. Exhausters. 

10. Housing; Dimensions of Housings; Shaft. 

11. Cone Wheels. 

12. Disk Fans; Number of Revolutions per Minute; Capacity of 
a Disk Fan; Horse Power Required. 

13. Choosing a Fan. Index. 

Twenty-two tables have been prepared and they have been ar- 
ranged in the way, which the experience of the author in designing 
heating and ventilating plants has shown to be the most convenient . 
The tables are full and complete, all calculations having been very 
carefully checked, read and revised. XIV. + 206 pages, 39 dia- 
grams. Full limp leather pocket book. 

Round Corners, gilt edges. $5,004 



CHARTS FOR 



Loi Pressure Steam Heating 

FOR THE USE OF 

ENGINEERS, ARCHITECTS, CONTRACTORS AND 
STEAM FITTERS. 

By J. H. KINEALY, M.E. 

M. Am. Soc. M. E., M. Am. Soc. of H. and V. Eng'rs, <5rV\, &c. 



The author has long been in the habit of using charts to aid him 
in his work. Knowing the value of them in saving time, simplifying 
work an i ensuring correct calculations he feels confident that they 
will be appreciated by engineers, architects and contractors, for whose 
benefit they have been compiled. Care has been taken to make the 
charts as clear and as easily understood and, above all, as accurate m, 
possible. They have been based upon theoretical considerations, 
modified by what is considered to be good practice in this country. 



Chart i. —This chart is for determining the number of square feet 
of heating surface of a low pres>ure steam heating system, pressure 
not to exceed 5 lbs. per square inch by the gauge, necessary to 
supply the heat lc>t through the various kinds of wall surtaces of 
rooms. The chart is divided into four parts Chart 2. — For detei- 
mining the diameters of the supply and return pipes for a heating 
system Chart 3. — For finding the number of square feet of boiler 
heating surface and the numbei of square feet of grate surface for 
a boiler that is to supply steam to a steam heating system. Chart a. — 
For determining the area of the cross section of a square flue, or the 
diameter of a round flue, leading from an indirect radiation heater to 
the register in a room to be heated. 

Full details are given for the use of these cards 

These four charts are printed on heavy white card-board and bound 
together with cloth, size 13 in. by 93^ in., $|.00t. 

These cards are securely packed for mail and sent to any part of 
the World on receipt of price. 



Gas Analyst's Manual. 

By JAQUES ABADY, M. Inst. Mech. E. 

{Incorporating F. W. Hartley's "Gas Analyst's Mannar 1 and 
"Gas Measurement.)" 



Extract from Preface. 



The numerous requests received by the Publishers for the late 
Mr. F. W. Hartley's "Gas Analyst's Manual and "Gas Measure- 
ment" form the justification of the present work, which embodies 
practically the entire contents of those two volumes. It has been 
found, however, that their scope was too narrow to comply with 
modern requirements in various directions, although ample at the 
time they were written, and so I have ventured to add such ex- 
tensions as appeared to be necessary in order to meet the demand 
which exists for a comprehensive work on Gas Apparatus and its 
use. 

This large work has been in course of preparation for the past 
three years by Mr. Jaques Abady, and has been very carefully 
revised by other experts. 

Many valuable tables of data have been included, a number of 
which come from the private note books of the Author, being 
practically results obtained by him during many years of work as 
Expert, Gas Engineer and Gas-Works Materials Manufacturer. 

CONTENTS OF CHAPTERS. 

1. — Photometry (58 pages. 

2. — The table photometer and Photcmeter Room (38 pages.) 

3. — Standard of Light (32 pages.) 

4 — Calorimetry and Specific Gravity, with a note on Mond Gas (48 pages ] 

5. — The Referees' Test for Sulphur and Ammonia in Gas (28 pages ) 

6.— Coal Testing (22 pages,) 

7. — Testing Enrichment and Purification Materials (33 pages ) 

8. — Purity Tests for Gas in the Various Stages of its Manutacture (43 pages.) 

9. — Testing Bye-products (35 pages.) 
10. — Technical Gas Analysis (63 pages.) 
11. — Meter-Testing Apparatus (48 pages.) 
12. — Meter and Governor Testing (34 pages,) 
.Appendix, Data, Tables, Formulae, etc., (38 pages). 

And very complete Contents, Index and List of Illustrations, and Tables, &c. &o. 
XV+560 pages, 5 J x 8 \ in., 93 illustrations and 9 folding plates 

Bound in Handsome Half Leather - $6.50 % 



The Design and Construction 

OF 

Oiix Engines. 



WITH FULL DIRECTIONS FOR 



erecting, testing, installing, Running ana Repairing. 

Including descriptions of American and English 

KEROSENE OIL ENGINES. 



By A. H. GOLDINGHAM, M.E. 



Synopsis of Contents of Chapters: 

i. Introductory ; classification of oil engines ; vaporizers ; ignition 
and spraying devices ; different cycles of valve movements. 2. On 
design and construction of oil engines ; cylinders ; crankshafts ; con- 
necting rods ; piston and piston rings ; fly-wheels ; air and exhaust 
cams, valves and valve boxes ; bearings ; valve mechanism, gearing 
and levers ; proportions of engine frames ; oil-tank and filter ; oil 
supply pipes ; different types of oil engines ; cylinders made in more 
than one piece ; single cylinder and double cylinder engines ; crank- 
pin dimensions ; fitting parts ; assembling of oil engine ; testing 
water jackets, joints, etc. 3. Testing for leaks, faults, power, 
efficiency, combustion, compression ; defects as shown by indicator ; 
diagrams for setting valves ; how to correct faults; indicator fully 
described ; fuel consumption test, etc. 4. Cooling water tanks ; 
capacity of tanks ; source of water supply ; system of circulation ; 
water pump ; exhaust silencers ; self starters ; utilization of waste 
heat of exhaust 5. Oil engines driving dynamo; installation of 
plant ; direct and belt connected ; belts ; power for electric lighting ; 
loss of power. 6. Oil engines driving air compressors ; direct con- 
nected and geared ; table of pressures ; pumping outfits ; oil engines 
driving ice and refrigeration outfits. 7. Full instructions for run- 
ning different kinds of oil engines. 8. Hints on repairs ; adjustment 
of crank-shaft and connecting rod bearing ; testing oil inlet valves 
and pump ; fitting new spur gears, etc. q. General descriptions with 
illustrations of American and English oil engines ; methods of work- 
ing ; portable oil engines, etc., etc. Index and tables. ^ 

XIII. + 196 pages, 7i z 5i, 79 illustrations, cloth, $2.00 



PRACTICAL HANDBOOK 

ON 

GAS ENGINES, 

With Instructions for Care and Working of the $vme. 
By G. LIECKFELD, C.E. 

TRANSLATED WITH PERMISSION OF THE AUTHOR BY 

Geo. Richmond, M.E. 

TO WHICH HAS BEE>f ADDED FULL DIRECTIONS FOR THE RUNNING O? 

OIL ENGINES. 



CONTENTS. 

Choosing and installing a gas engine. The construction of good 
gas engines. Examination as to workmanship. As to running. A** 
to economy. Reliability and durability of gas engines. Cost of in- 
stalling a gas engine. Proper erection of a gas engine. Construc- 
tion of the foundation. Arrangement for gas pipes. Rubber bag 
T Peking devices. Exhaust pipes. Air pipes. Setting up gas en- 
g*. es. Brakes and their use in ascertaining the power of gas en- 
gines. Theory of the brake. The Brauer band brake. Arrange 
ment of a brake test. Explanation of the expressions " Brak 
Power" and 44 Indicated Power." Comparisons of the results of the 
brake test and the indicated test. Quantity of work consumed bv 
external friction of the engine Distribution of heat in a gas engine 
Attendance on gas engines. General remarks. Gas engine oi. 
Cylinder lubricators Rules as to starting and stopping a gas engine 
The cleaning of a gas engine. General observations and specific ex* 
amination for defects. Different kinds of defectives. The engine 
refuses to work. Non-starting of the engine. Too much pressure 
on the gas. Water in the exhaust pot. Difficulty in starting the en- 
gine. Clogged slide valve. Leaks in gas pipes. Unexpected 
stopping of engine Irregular running. Loss of power. Weak g as 
mixtures. Late ignition. Cracks in air inlet. Backfiring Knock- 
ing and pounding inside of engine. Dangers and precautionary 
measure in handling gas engines. Examination of gas pipes. Pre- 
cautions when :— Opening gas valves. Removing piston from cylin- 
der. Examining with light openings of gas engines. Dangers in 
starting. Dangers in cleaning. Safeguards for fly-wheels. Danger 
of putting on belts. Oil Engines. Gas engines with producer 
gas. Gasoline and oil engines The " Hornsby-Akroyd ' oil engine. 
Failure to start. Examination of engine in detail. Vaporizer valv* 
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THE CMTRY OF FIRE 



-AND- 



FIRE PREVENTION. 



A HANDBOOK FOR INSURANCE SURVEYORS. 
WORKS MANAGERS, AND ALL INTERES- 
TED IN FIRE RISKS AND 
THEIR DIMINUTION 

BY 

HERBERT INGLE, F.I.C., F.C.S. 

—AND— 

HARRY INGLE, Ph.D., B.Sc. 

TECHNOLOGICAL CHEMIST. 



Contents of Chapters. 

I. Definition of Fire, Old Theories as to its Nature, Modern Views 
of Combustion— The Physical and Chemical Properties of the Atmos- 
phere, the Chief Properties of its Constituents — Some Conditions 
Affecting the Combustion of Substances in Air, the Principle of the 
Miners Safety Lamp. 

II. Explanation of Chemical Terms, Outline of the Atomic Theory. 
Brief Explanations of the Use of Chemical Formulae and Equations. 

III. Methods of Preparations of Oxygen, Brin's Oxygen Manufac- 
ture — Heat Measurements, the Calorimeter, Calorific Power of Sub- 
stances Burning in Air. 

IV. Coal Gas : Its Preparation, Purification and Composition— 
Properties of Its Chief Constituents— Reciprocity of Combustion- 
Gaseous Diffusion — Explosion of Gases — Dust Explosions. 

V. Fuel : Chemical Composition of Wood, Charcoal, Peat, Lig 
trite, Coal, Coke, Petroleum, Coal Gas— Use of "Atmospheric BtiT 
ners " — Producer Gas — Water Gas— Dawsoo Gas, 



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METAL WORKING TOOLS AND THEIR USES. A Handbook 
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SIMPLE MECHANICAL WORKING MODELS. How to make 
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MODEL STEAMER BUILDING. A practical handbook on the 
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MACHINERY FOR MODEL STEAMERS. On the design, con- 
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THE SLIDE VALVE. Simply explained for working engin- 
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GAS AND OIL ENGINES. A practical handbook on, with in- 
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STANDARD SCREW THREADS. A Guide to Standard Screw 
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STEAM TURBINES, How to design and build them. A prac- 
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HARD WOODS, ENGLISH AND FOREIGN A practical de- 
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^5?o> 



PONS' 

]VTecl)ai)ics Qwij^^ook 

A WORK THAT SHOULD BE IN YOUR BOOKCASE. 

The general method of treatment of each subject, is first 
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THE FOLLOWING ARE THE PRINCIPAL CONTENTS. 

Mechanical Drawing, (13 pages.) 

Mechanical Movements, (55 pages.) 

Casting and Founding in Brass and Bronze, (30 pages.) 

Forging and Finishing, (46 pages.) 

Soldering in all its branches, (26 pages.) 

Sheet Metal Working, (10 pages.) 

Turning and Turning Lathes, (31 pages.) 

Carpentry, (224 pages.) 

Log Huts, Building, Etc., (8 pages.) 

Cabinet-Making, (36 pages.) Upholstery, (6 pages.) 

Can ingand Fretwork, (13 pages.) 

Pic-ure Frame Making, (4 pages.) 

Pointing, Graining and Marbling, (28 pages.) 

Scaining, (13 pages.) Gilding, (3 pages.) 

Polishing, (23 pages.) Varnishing, (4 pages.) 

Paper Hanging, (4 pages.) Glazing, (7 pages.) 

Plastering and White Washing, (9 pages.) 

Lighting, (8 pages.) 

Foundations and Masonry, (46 pages.) 

Roofing, (14 pages.) 

Ventilating and Warming, (13 pages.) 

Electric Bell and Bell Hanging, Gas Fitting, (8 pages.) 

Roads and Bridges, Banks, Hedges, Ditches and Drains, As- 
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ffotal number of pages 702. Total number illustrations 1,420 

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Nitro-Glycerine, Photography, Pottery, Varnishes, etc., etc. 420 pages, 103 
illustrations, 12mo, cloth, $2.00. 

CborTinrl ^P^f-i^c Principal Contents.- Acidimetry, Albumen. 

J7CCU11U -^^1 1C^. Alcohol, Alkaloids, Bitters, Bleaching, Boiler 
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Extracts, Fire-proofing, Glycerine. Gut, Iodine, Ivory Substitutes. Leather, 
Matches Pigments, Paint, Paper, Parchment, etc., etc. 485 pages, 16 illustra- 
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Nickel, Platirum, Silver, Slag, Tin, Uranium, Zinc, etc., etc. 480 pages, 133 
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rtfUrLIl OCI 1C». an( j Stowing, Embalming and Preserving 
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Distilling, Emulsifying, Evaporating, Filtering, Percolating and Macerating 
Electrotyping, Stereotyping. Book-binding, Straw-plaiting, Musical Instru- 
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SPONS' ENCYCLOPEDIA 

OF THE 

Industrial Arts, Manufactures 



AND 



Commercial Products. 

EDITED BY 

Q. Q. ANDRE, F.Q.S., Asso.=M. Inst. C.E. 

AND 

C. Q. WARNFORD LOCK, F.L.S., F.Q.S., M.I.M.M. 

Assisted by many prominent Manufacturers, Chemists and Scientists. 



This encyclopedia is written by practical men for practical men. 

Raw Materials form perhaps its most important feature and are 
dealt with in a way never before attempted. 

Manufacturers are discussed in detail from the manufacturing 
standpoint by manufacturers of acknowledged reputation. 

Special consideration is given to the utilization of waste, the pre- 
tention of nuisance, and the question of adulterations. 

Technicalities are explained, and bibliographies (English, Amen 
;can, French, German, etc.), are appended to the principal articles. 
( Over 2,000 pages and nearly 2,000 illustrations. 

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Dubelle's Famous Formulas. 

KNOWN AS 

Non Plus Ultra Soda Fountain Requisites of Modern Times 

By U H. 1)1 BELLE 

A practical Receipt Book fot Druggists, Chemists, Confectioners and Venders 

of Soda Wmter. 

S.'J*OPSIS OF CONTENTS. 

Introduction. — Notes on natural fruit juices and improved me- 
thods for their preparation. Selecting the fruit. Washing and 
pressing the fruit. Treating the juice. Natural fruit syrups and 
mode of preparation. Simple or stock syrups. 

FORMULAS. 

Fruit Syrups.— Blackberry, black current, black raspberry, ca- 
tawba, cherry, concord grape, cranberry, lime, peach, pineapple, 
plum, quince, raspberry, red current, red orange, scuppernong grape, 
strawberry, wild grape. New Improved Artificial Fruit Syrups. — 
Apple, apricot, banana, bitter orange, blackberry, black current, 
cherry, citron, curacoa, grape, groseille, lemon, lime, mandarin, mul- 
berry, nectarine, peach, pear, pineapple, plum, quince, raspberry, 
red current, strawberry, sweet orange, tangerine, vanilla. Fancy 
Soda Fountain Syrups. — Ambrosia, capillaire, coca-kina, coca van- 
illa, coca-vino, excelsior, imperial, kola coca, kola-kina, kola-vanilla, 
kola-vino, nectar, noyean, orgeat, sherbet, syrup of roses, syrup of 
violets, a Artificial Fruit Essences. — Apple, apricot, banana, berg- 
amot, blackberry, black cherry, black currant, blueberry, citron, 
cranberry, gooseberry, grape, lemon, lime fruit, melon, nectarine, 
orange, peach, pear, pineapple, plum, quince, raspberry, red currant, 
strawberry. Concentrated Fruit Phosphates. -Acid solution of 
phosphate, strawberry, tangerine, wild cherry. — 29 different formulas. 
New Malt Phosphates — 36. Foreign and Domestic Wine Phos- 
phates — 9. Cream-Fruit Lactarts — 28. Soluble Flavoring Ex- 
tracts and Essences — 14. New Modern Punches— 18. Milk 
Punches — 17. Fruit Punches— 32. Fruit Meads — 18. New Fruit 
Champagnes — 17. New Egg Phosphates — 14. Fruit Juice Shakes 
— 24. Egg Phosphate Shakes. Hot Egg Phosphate Shakes. 
Wine Bitter Shakes — 12. Soluble Wine Bitters Extracts — 12. 
New Italian Lemonades — 18. Ice Cream Sodas— 39. Non-Poison- 
ous Colors. Foam Preparations. Miscellaneous Formulas— 26. 
Latest Novelties in Soda Fountain Mixtures— 7. Tonics. — Beef, 
iron and cinchona; hypophosphite ; beef and coca ; beef, wine and 
iron ; beef, wine, iron and cinchona ; coca and calisaya. Lactarts. 
— Imperial tea ; mocha coffee ; nectar; Persian sherbert. Punches. 
Extracts. — Columbia root beer ; ginger tonic ; soluble hop ale. 
Lemonades. — French ; Vienna. Egg nogg. Hop ale. Hot torn. Malt 
wine. Sherry cobbler. Saratoga milk shake. Pancretin and wine. 
Kola-coco cordial/ Iron malt phosphate. Pepsin, wine andiron, etc 
157 Pages, Nearly 500 Formulas. 12mo, Clot*. SI 



Latest practice, New and original cuts. 



Katharine Mellishs 
COOKERY 

AND 

DOHEST1C MANAGEHENT. 



SYNOPSIS OF CONTENTS. 

Complete Breakfast Menus, with receipts, pages 1 — 27. 

Complete Luncheon Menus, with receipts, pages 28 — 74. 

Tea Menus, High Tea Menus, with receipts, pages 75 — 124. 
Complete Dinner Menus, with receipts, pages 125 — 220. 

processes : — This chapter is illustrated with exact position of the 
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etc. Larding and icing, etc. 

Receipts of separate dishes. 

Stocks and soups of all kinds, broth, gravy, etc. 

Fish. — To clean, general rules for cooking. Numerous receipts 
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The preparation and making of gravies, stuffing, forcemeats, 
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Entrees. — Their preparation and serving, with receipts. 

Joints. — Preparation, cooking and serving of, with many receipts. 

Vegetables. — Preparation and cooking of plain and dressed vege- 
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Poultry and game. — Preparation, cooking and serving, with receipts. 

Pastries, pudding and sweet dishes. Cakes, biscuits, bread. 

Omelets. — With receipts for preparing, cooking and serving. 

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987 pages, 56 full page colored plates, 439 illus. Size, 7iin. x 
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